RA 


LABORATORY  TEXT-BOOK  OF 
EMBRYOLOGY 


MINOT 


BY  THE  SAME  AUTHOR 

A  Bibliography  of  Vertebrate  Embryology 

Containing  over  three  thousand  titles  classi- 
fied by  subjects  and  indexed  by  authors. 
Quarto.  Price,  $2.50. 


Human  Embryology 

8vo,  pp.  xxiii,  815.     With  four  hundred  and 
sixty-three  illustrations.     Price,  $6.00. 


A 


LABORATORY  TEXT-BOOK 


OF 


EMBRYOLOGY 


CHARLES  SEDGWICK  MINOT,  LLD.  (Yale),  D.Sc.  (Oxford) 

PHOFESSOR   OF    HISTOLOGY    AND    HUMAN   EMBRYOLOGY   IN   THE    HARVAKL)    MEDICAL  SCHOOL 


WITH  218  ILLUSTRATIONS,  CHIEFLY  ORIGINAL 


PHILADELPHIA 

P.    BLAKISTON'S    SON    &    CO. 

1012  WALNUT  STREET 
1903 


BIOLOGY 

RA 
G 


COPYRIGHT,    1903, 
BY  CHARLES  SEDGWICK  MINOT. 


PRESS   OF  WM.  F.  FELL  &  CO.. 

1220-24  SANSOM  ST., 

PHILADELPHIA. 


TO 

HENRY  PICKERING  BOWDITCH 

AS  A  TOKEN  OF 

ADMIRATION  AND  LONG  FRIENDSHIP  THIS  VOLUME 

IS  DEDICATED  BY  THE 

AUTHOR 


258362 


PREFACE. 


The  accompanying  volume  is  intended  primarily  for  the  use  of  students, 
taking  a  practical  laboratory  course  in  Embryology.  The  author's  experience 
has  led  him  to  believe  that  the  study  of  carefully  selected  sections  of  embryos, 
accompanied  by  directions  and  explanations  of  the  significant .  structures  in 
each  section,  offers  many  advantages.  This  conviction  has  determined  the 
arrangement  of  the  book.  Attention  is  given  chiefly  to  such  points  as  serve 
to  explain  adult  anatomical  relations,  to  illustrate  general  biological  principles, 
and  to  afford  insight  into  pathological  processes. 

Portions  of  the  text  and  many  of  the  figures  have  been  borrowed  from  the 
author's  "  Human  Embryology."  The  woodcuts  in  Chapter  IV  were  made  by 
C.  L.  Albert  Probst,  of  Braunschweig,  after  drawings  by  Dr.  E.  A.  Locke.  To 
both  of  these  artists  the  author  is  indebted  for  their  beautiful  work.  Much 
assistance  has  been  rendered  by  Dr.  F.  T.  Lewis,  of  the  Harvard  Medical  School, 
to  whom  special  acknowledgments  are  due  for  the  reconstructions  of  the  anat- 
omy of  the  pig  embryo  of  twelve  millimeters  and  for  invaluable  help  in  the 
correction  of  the  proofs. 

Many  of  the  illustrations  are  from  the  Harvard  Embryological  Collection, 
without  which  this  work  could  not  have  been  undertaken.  The  number  of  the 
embryo  and  of  the  section  is  given  for  all  such  illustrations. 

The  title  was  suggested  by  Dr.  W.  T.  Porter's  "A  Laboratory  Text-book 
of  Physiology,"  and  is  adopted  with  his  approval. 

The  author  requests  those  who  use  this  book  to  communicate  to  him  any 
suggestions,  which  their  experience  may  lead  to,  for  improving  it,  in  case  it 
meets  with  sufficient  favor  to  call  for  a  new  edition. 

CHARLES  SEDGWICK  MINOT. 

CORTINA  D'AMPEZZO,  August,  1902. 


CONTENTS. 


PAGE 

CHAPTER  I. — GENERAL  CONCEPTIONS, 17-43 

The  Vertebrate  Type  of  Structure, 18 

The  Principal  Modifications  of  the  Vertebrate  Type, 22 

Definition  of  Anlage, 25 

A  Summary  of  Embryological  Development, 25 

Cytomorphosis, 27 

Comparison  of  Larval  and  Embryonic  Types  of  Development, 32 

Germ-layers, 34 

The  Relations  of  Surface  to  Mass, , 37 

The  Law  of  Unequal  Growth, 38 

Germ-cells, 39 

The  Theory  of  Heredity, 40 

The  Law  of  Recapitulation, 41 

CHAPTER  II. — THE  EARLY  DEVELOPMENT  OF  MAMMALS, 44-111 

The  Spermatozoon, •  ...  44 

The  Fully  Grown  Ovum  before  Maturation, .  45 

Ovulation,  46 

The  Maturation  of  the  Ovum, 47 

Impregnation  of  the  Ovum, 49 

Segmentation  of  the  Ovum, 54 

The  Blastodermic  Vesicle, 60 

The  Embrvonic  Shield, 62 

Origin  of  the  Mesoderm, 63 

The  Primitive  Axis, 65 

The  Notochordal  Canal, 66 

The  Notochord, 67 

The  Ultimate  Fate  of  the  Notochord, 68 

The  Origin  of  the  Nervous  System, 69 

The  Structure  of  the  Medullary  Canal, , 71 

The  Early  History  of  the  Mesoderm, 74 

Somatopleure  and  Splanchnopleure, "j6 

The  Embryonic  Coelom, 79 

The  Mesenchyma, 83 

The  Germ-cells, 84 

The  Yolk-sac, 85 

The  Origin  of  the  Blood-vessels  and  Blood, 90 

The  Blood-corpuscles, 93 

The  Origin  of  the  Heart, 96 

The  Germinal  Area,  97 

The  Main  Vessels  of  the  Area  Vasculosa, 97 

The  Liver,  loo 

The  Oral  and  Anal  Plates, IOO 

The  Excretory  Organs, IOI 

The  Allantois, 103 

The  Trophoblast, 106 

The  Growth  of  the  Embryo, 107 

The  Umbilical  Cord, : 109 

CHAPTER  III. — THE  HUMAN  EMBRYO,  , 112-156 

Calculation  of  the  Age  of  the  Human  Embryo, 112 

The  Classification  of  the  Early  Stages, 113 

ix 


x  CONTENTS. 

PAGE 

Hypothetical  Development  of  the  Blastodermic  Vesicle  in  Primates 116 

Relationsof  the  Embryo  to  the  Uterus, Il& 

Ovum  of  a  Monkey  in  the  Second  Stage, - I21 

Human  Embryo  in  the  Second  Stage I23 

Embryo  of  a  Gibbon  in  the  Third  Stage I27 

Human  Embryo  in  the  Fourth  Stage  with  Medullary  Plate, 129 

Human  Embryo  in  the  Fifth  Stage  with  Open  Medullary  Groove, I31 

Human  Embryo  in  the  Sixth  Stage  with  Medullary  Canal I32 

Human  Embryo  in  the  Seventh  Stage  with  One  Gill-cleft, 135 

Human  Embryo  in  the  Eighth  Stage  with  Two  Gill-clefts *35 

Human  Embryo  in  the  Ninth  Stage  with  Three  Gill-clefts, 138 

Human  Embryo  in  the  Tenth  Stage  with  Four  Gill-clefts, I41 

Human  Embryo  in  the  Eleventh  Stage  with  Limb-buds H2 

Human  Embryo  of  Twenty-six  Days, H3 

Human'Embryo  of  Twenty-eight  Days, 143 

Embryos  of  the  Second,  Third,  and  Fourth  Months 145 

CHAPTER  IV. — STUDY  OF  PIG  EMBRYOS, 157-268 

Methods  of  Obtaining  Embryos, *57 

The  Making  of  Serial  Sections, 158 

Selection  of  the  Planes  of  Section  and  the  Stages  for  Practical  Study, 158 

The  Study  of  the  External  Form, 159 

Pig  Embryo  of  10  mm  , 160 

Pig  Embryo  of  12  mm.     General  Anatomy, 162 

Pig  Embryo  of  15  mm.     (External  Form), , 17° 

Pig  Embryo  of  20  mm.     (External  Form), I71 

Pig  Embryo  of  12  mm.     (Studied  in  Sections),    .    .        173 

The  Study  of  Transverse  Sections, 1 75 

Section  through  the  Upper  Part  of  the  Otocyst,     .    .    .' 175 

Section  through  the  Lower  Part  of  the  Otocyst, 181 

Section  through  the  First  Gill-cleft, 184 

Section  through  the  Second  Gill-cleft, 187 

Section  through  the  Third  Gill-cleft 190 

Section  through  the  Fourth  Gill -cleft, 193 

Section  through  the  Anterior  Limbs  and  Heart, 195 

Section  to  show  the  Brachial  Plexus,      199 

Section  through  the  Stomach  and  Liver, 201 

The  Study  of  Sagittal  Sections, 205 

Median  Section  of  the  Head, 205 

Section  of  the  Head  through  the  Principal  Ganglia, 208 

The  Study  of  Frontal  Sections, 211 

Section  through  the  Trigeminal  Roots, 212 

Section  through  the  Acustico-facial  Ganglion, • .  213 

Section  through  the  Otocyst 214 

Section  through  the  Dorsal  Vertebrae, 215 

Pig  Embryo  of  9  mm.     (Studied  in  Sections), 217 

Transverse  Section  through  the  Branchial  Arches, 217 

Sagittal  Section  to  the  Right  of  the  Median  Plane, 219 

Frontal  Section  through  the  Mid-brain  and  Fore-brain,  .    . 222 

Frontal  Section  through  the  Umbilical  Opening, 223 

Frontal  Section  through  the  Second  and  Third  Gill-clefts, 227 

Pig  Embryo  of  6  mm.     (Studied  in  Sections), 228 

Pig  Embryo  of  17  mm.  (Studied  in  Sections), 231 

Transverse  Section  through  the  Lungs, 231 

Section  through  the  Wolffian  Body  and  Genital  Gland, 235 

Section  through  the  Kidney, 237 

Frontal  Section  of  the  Umbilical  Cord, 239 

Pig  Embryo  of  20  mm.     (Studied  in  Sections), 240 

Transverse  Section  through  the  Snout, 240 

Transverse  Section  through  the  Lower  Part  of  the  Neck, 241 

Transverse  Section  through  the  Lungs, 244 

Transverse  Section  through  the  Posterior  Limbs, 247 

Transverse  Section  through  the  Mammary  Anlage, 249 


CONTENTS.  xi 

PAGE 

Sagittal  Section  through  the  Right  Lung  and  Kidney, 250 

Frontal  Sections  of  the  Head, 253 

Pig  Embryo  of  24  mm.     (Studied  in  Sections), 259 

Frontal  Section  through  the  Eye 259 

Median  Sagittal  Section, 263 

CHAPTER  V. — STUDY  OF  YOUNG  CHICK  EMBRYOS, 269-304 

Method  of  Obtaining  Embryos, 269 

Embryo  Chick  with  Twenty-four  Segments, , 272 

Embryo  Chick  with  Twenty-eight  Segments, 274 

The  Study  of  Transverse  Sections, 274 

Horizontal  Section, •    •  290 

Histological  Differentiation  of  Chick  Embryo  with  Three  Gill-clefts, 293 

Embryo  Chick  with  Seven  Segments, 295 

Examination  in  the  Fresh  State, 295 

Examination  after  Hardening, 297 

Comparison  with  a  Rabbit  Embryo, 297 

Longitudinal  Section, 297 

Study  of  Transverse  Sections, 299 

CHAPTER  VI. — STUDY  OF  THE  BLASTODERMIC  VESICLE  AND  THE  SEGMENTATION  OF  THE  OVUM,  .  305-315 

Method  of  Obtaining  Blastodermic  Vesicles  from  the  Rabbit, 305 

Study  of  Rabbit  Blastodermic  Vesicles  in  Alcohol, 306 

The  Maturation,  Fertilization,  and  Segmentation  of  the  Ovum  in  White  Mice, 312 

The  First  Polar  Globule,       3*3 

The  Second  Polar  Globule, 3*3 

The  Single  Polar  Globule, 3'4 

Fertilization, 3I5 

CHAPTER  VII. — STUDY  OF  THE  UTERUS  AND  THE  FCETAL  APPENDAGES  IN  MAN, 316-355 

Histology  of  the  Uterus, 316 

Menstruation, 3*6 

Decidua  Menstrualis, 317 

The  Pregnant  Uterus:  the  Two  Stages, 319 

The  Human  Uterus  Three  Months  Pregnant, 319 

Human  Uterus  Seven  Months  Pregnant, 321 

Decidua  Vera  of  the  First  Stage  in  Section, 322 

Decidua  Reflexa  of  the  First  Stage, 32S 

Decidua  Vera  and  Chorion  Lseve  of  the  Second  Stage, 327 

The  Placenta  in  Situ,    . 329 

Decidua  Serotina  at  Seven  Months, 334 

The  Human  Placenta,  ...        33° 

Histology  of  the  Human  Chorion, 34 l 

The  Chorion  with  Trophoblast 342 

The  Chorionic  Villi, 345 

The  Structure  of  the  Amnion, 349 

The  Umbilical  Cord,     .    .  _ 351 

The  Structure  of  the  Human  Yolk-sac, 354 

CHAPTER  VIII. — METHODS, 356-37° 

Measuring  the  Length  of  Embryos, .• 356 

Methods  of  Reconstruction, 356 

Reconstruction  of  Drawings, 356 

Reconstruction  with  Wax-plates, 35^ 

Directions  for  Orienting  Serial  Sections, 36° 

Microtomes, •    •    •    •  36° 

Methods  of  Hardening  and  Preserving, 363 

Preservation  in  Alcohol, 365 

Directions  for  Imbedding  Specimens  to  be  Microtomed, 365 

Method  of  Mounting  Paraffin  Sections, 3^    • , ^66 

Methods  of  Staining, 367 

INDEX, 371 


LIST  OF  ILLUSTRATIONS. 


FIG-  PAGE 

1.  Human   Spermatozoa,  (after  Retziiis}, 44 

2.  Full-grown  Human  Ovum  before  Maturation,  (after  W.  Nagel}, 46 

3.  Ovum  of  a  Worm  (Sagitta)  with  Two  Pro-nuclei.     Around   Each  Pro-nucleus  is  shown  the  Aster 

(after  O.  Her  twig}, 52 

4.  Ovum  of  a  Rabbit,  Seventeen  Hours  after  Coitus,  with  the  Pro-nuclei  about  to  Conjugate  (after  Rein),  52 

5.  Ovum  of  White  Mouse.      Beginning  of  the  Conjugation  of  the  Pro-nuclei  (after  Sobotta}, 53 

6.  Ovum  of  White  Mouse.     Conjugation  of  the  Pro-nuclei,  and  Formation  of  the  Segmentation  Spindle 

(after  Sobotta}, 53 

7.  Ovum  of  White  Mouse.     First  Segmentation  Spindle  with  the  Chromosomes  of  the  Pro-nuclei  still 

forming  Two  Distinct  Groups  (after  Sobotta}, 53 

8.  Ovum  of  a  Rabbit  of  Twenty-four  Hours  (after  Coste), 55 

9.  Ovum  of  a  Snail  (Limax  campestris)  during  the  First  Cleavage.     The  Envelopes  of  the  Ovum  are 

not  Drawn  in.  (after  E.  L.  Mark], 55 

10.  Ovum  of  White  Mouse.     First  Segmentation  Spindle  with  Equatorial  Plate  of  Chromosomes  (after 

Sobotta), 56 

11.  Ovum  of  White  Mouse.      First  Segmentation  Spindle  (after  Sobotta}, 56 

12.  Ova  of  White  Mouse  with  Two  Segmentation  Spheres  or  Cells  (after  Sobotta}, 57 

13.  Ovum  of  a  Bat  ( Vespertilio  murina)  with  Four  Segmentation  Spheres  (after  Tan  Beneden  and  Julin),  58 

14.  Ovum  of  a  Virginian  Opossum,  with  Four  Segments  (after  Emil  Selenka}, 58 

15.  Rabbit's  Ovum  of  about  Seventy  Hours  (after  E.  van  Beneden), 59 

16.  Young  Blastodermic  Vesicle  of  a  Mole  (after  W.  Heape}, 59 

17.  Sections  through  the  Inner  Mass  of  Blastodermic  Vesicles  of  the  Mole  at  Three  Successive  Stages 

(after  W.  Heape}, 61 

18.  Transverse  Section  through  the  Embryonic  Shield  of  the  Blastodermic  Vesicle  of  a  Dog  of  Eleven 

or  Fifteen  Days  (Precise  Age  Unknown)  (after  Bonnet}, 62 

19.  Surface  View  of  the  Embryonic  Shield  of  the  Blastodermic  Vesicle  of  a  Dog  of  Thirteen  to  Fifteen 

Days  (Precise  Age  Unknown)  (offer  Bonnet}, 63 

20.  Central  Portion  of  a  Sheep's  Blastodermic  Vesicle  of  Twelve  to  Thirteen  Days  (after  Bonnet},  .    .    .  64 

21.  Blastodermic  Vesicle  of  a  Rabbit  of  Seven  Days.     Portion  of  the  Mescderm   of  the  Area  opaca 

(after  Kolliker'}, • 64 

22.  Germinal  Area  of  a  Guinea-pig  at  Thirteen  Days  and  Twenty  Hours,  seen  from  the  Under  (Ento- 

dermal)  Side, 66 

23.  Transverse  Section  of  a  Mole  Embryo  (Heape's  Stage  H)  (after  W.  Heape}, 67 

24.  Degenerating  Tissue  of  the  Notochord  from  the  Central  Portion  of  the  Intervertebral  Disc  of  a  Cow's 

Embryo  (after  Leboucq), 69 

25.  Surface  View  of  the  Embryonic  Shield  of  a  Dog  Embryo,  with  Medullary  Plate, 69 

26.  Cross-section  of  a  Human  Embryo  of  1.54  mm.  (after  Count  Spee~}, 70 

27.  Section  of  a  Young  Cat  Embryo  and  of  the  Uterine  Wall  to  Which  it  is  Attached.     (Embryo  No. 

398,  Section  76), , *    72 

28.  Transverse  Section  of  a  Rabbit  Embryo  of  Eight  Days  and  Two  Hours, 73 

29.  Three  Diagrams  of  Embryonic  Areas  of  Chicks  to  show  the  Growth  of  the  Mesoderm  (after  Duvai),  75 

30.  Transverse  Section  of  an  Early  Stage  of  an  Axolotl  (after  Bellonci),      77 

31.  Generalized  Diagram  of  an  Amniote  Vertebrate  Embryo, 78 

32.  Transverse  Section  from  a  Chick  Embryo  with  about  Eighteen  Segments, 80 

33.  Section  of  a  Very  Young  Cat  Embryo.     (Transverse  Series  413,  section  181), 8l 

34.  Diagrammatic  Section  of  the  Yolk  of  a  Hen's  Egg  at  an  Early  Stage  to  show  the  Relation  of  the 

Primitive  Entodermal  Cavity, 85 

35.  Wall  of  the  Yolk-sac  in  the  Region  of  the  Area  opaca  of  a  Chick  of  the  Second  Day, 86 


xiv  LIST  OF  ILLUSTRATIONS. 


PAGE 


FIG. 

36.  Wall  of  the  Yolk-sac  in  the  Region  of  the  Area  opaca  of  a  Rabbit  Embryo  of  Thirteen  Days,  .    .    . 

37.  Section  of  the  Yolk-sac  of  a  Young  Human  Embryo  (after  Keibel),       

38.  Human  Embryo,  2.15  mm.  Long  (after  W.  His), 

39.  Human  Embryo  of  2.  6  mm.  (after  W.  His), 89 

40.  Human  Embryo  of  9.8  mm.     Probable  Age  Thirty  Days, 9° 

41.  Section  of  the  Area  vasculosa  of  a  Chick  Embryo  of  the  Second  Day, 92 

42.  Development  of  Blood-corpuscles,         .    .                95 

43.  Diagrams  to  Illustrate  the  Separation  of  the  Embryo  from  the  Yolk, 96 

44.  Diagram  of  the  Circulation  in  a  Chick  at  the  End  of  the  Third  Day,  as  Seen  from  the  Under  (Ento- 

dermal)  Side, 9$ 

45.  Area  Vasculosa  of  a  Rabbit,  Presumably  of  about  Twelve  Days  (after  Van  Beneden  and  Julin),  99 

46.  Longitudinal  Section  of  the  Posterior  End  of  a  Sheep  Embryo  of  Sixteen  Days  (after  R.  Bonnet),  .  101 

47.  Frog  (Rana  temporaria)  Tadpole  of  12.0  mm.     Cross-section  of  the  Pronephric  Region  (after  M. 

Fiirbnnger), IO2 

48.  Diagrams  illustrating  the  Relations  of  the  Allantois  in  Unguiculate  Mammals, 104 

49.  Pig,  15.0  mm.,  Series  135,  Section  58,  to  Show  the  Relations  of  the  Chorion  to  the  Uterus,      .    .    .  106 

50.  Transverse  Section  of  an  Embryo  Catfish  (Amiurus) ;  Series  25,  Section  43, 108 

51.  Sections  of  Two  Human  Umbilical  Cords, no 

52.  Human  Embryo  at  the  Beginning  of  the  Third  Week, 114 

53.  Two  Diagrams  to  Illustrate  the  Hypothetical  Early  Development  of  Primates, 1 16 

54.  Diagram  of  an  Early  Stage  of  a  Primate  Embryo, 117 

55.  Semi-diagrammatic  Outline  of  an  Antero-posterior  Section   of   a-  Human    Uterus   Containing   an 

Embryo  of  about  Five  Weeks  (after  Allen  Thompson), 119 

56.  Human  Uterus,  about  Forty  Days  Advanced  in  Pregnancy  (after  Coste), 120 

57.  Blastodermic  Vesicle  of  a  Monkey  (Semnopithecus  nasicus)  Attached  to  the  Uterus  ;  Vertical  Sec- 

tion (after  E.  Selenka), 122 

58.  Embryo  of  the  Preceding  Figure  More  Highly  Magnified  (after  E.  Selenka), 123 

59.  Section  of  Peters's  Ovum  in  Situ  (after  H.  Peters), 124 

60.  Embryo  of  a  Gibbon  (Hylobates  concolor)  in  the  Third  Stage  (after  E.  Selenka), 126 

61.  Embryo  of  a  Gibbon,  Side  View  of  the  Embryo  of  Fig.  60  (after  E.  Selenka),    .    .        127 

62.  Transverse  Section  of  the  Embryo  of  the  Preceding  Figure  (after  E.  Selenka), 127 

63.  Surface  View  of  the  Embryonic  Area  of  the  Ovum  Shown  in  Fig.  61, 128 

64.  Reconstruction  of  a  Human  Embryo  1.54  mm.  Long  (after  Count  Spee), 129 

65.  Human  Embryo  of  1.54  mm.     Median  Section  from  a  Wax  Model  Reconstructed  from  Sections 

(after  Count  Spee), 130 

66.  Human  Embryo  of  1.54  mm.  (after  Count  Spee), 130 

67.  Human  Embryo  with  Open  Medullary  Groove  (after  W.  His),    . 131 

68.  Human  Embryo  of  from  Thirteen  to  Fourteen  Days  (after  J.  Kollmann), 132 

69.  Human  Ovum,  said  to  be  from  Fifteen  to  Eighteen  Days  Old, 133 

70.  Embryo  of  Fig.  69,  Separated  from  the  Yolk-sac  and  Viewed  from  the  Under  Side,  .......  134 

71.  Human  Embryo,  2. 15  mm.  Long  (after  W,  His),        .  135 

72.  Reconstruction  of  the  Anatomy  of  the  Embryo  Shown  in  Fig.  71  (after  W.  His), 136 

73.  Human  Embryo  of  2.6  mm.  Length  (after  W.  His), 137 

74.  Reconstruction  of  the  Anatomy  of  the  Embryo  of  2.6  mm.  in  Fig.  72  (after  W.  His), 138 

75.  Human  Embryo,  4.2  mm.  (after  W.  His), 139 

76.  Outline  of  the  Entodermal  Canal  of  a  Human  Embryo  of  4.2  mm.  (after  W.  His), 140 

77-   Reconstruction  of  the  Anatomy  of  a  Human  Embryo,  3.2  mm.   Long,  showing  the  Anterior  End 

Viewed  from  the  Ventral  Side,  ....            140 

78.  Reconstruction  of  the  Anatomy  of  the  Human  Embryo  of  4.2  mm.  Shown  in  Fig.  75  (after  W.  His),  141 

79.  Human  Embryo  of  About  Twenty-three  Days  (after  W.  His), 142 

80.  Human  Embryo  of  7  mm.  (after  F.  P.  Mall), 142 

81.  Human  Embryo  of  7.5  mm.  in  Maximum  Length  (after   IV.  His), 143 

82.  Reconstruction  of  the  Pharyngeal  Region  of  a  Human  Embryo  of  11.5  mm.  (after  W.  His),    .    .    .  144 

83.  Human  Ovum  with  Embryo  of   9.8  mm.     The  Chorion  Has  Been  partly  Removed  to  Show  the 

Embryo  (Minot  Collection,  275) ' 145 

84.  Embryo  of  the  Preceding  Figure, 146 

85.  Human  Embryo  of  n  mm.  (after  W.  His), ^ 147 

86.  Human  Embryo  of  about  14  mm., 147 

87.  Human  Embryo  of  about  Thirty-five  Days  (after  Coste),     . 148 

88.  Human  Embryo  of  about  1 6  mm.  (after  W.  His), 149 

89.  Human  Embryo  of  22  mm., 150 

90.  Human  Embryo  of  28  mm., 151 


LIST  OF  ILLUSTRATIONS. 


xv 


91. 

92. 
93- 
94- 
95- 
96. 

97- 
98. 
99. 
100. 

101. 

102. 
103. 

104. 
105. 

106. 
107. 

108. 
109. 

no. 
in. 

112. 

US- 
114. 

"5- 
116. 
117. 
n8. 

119. 

120. 
121. 

122. 
123. 
124. 
125. 
126. 
127. 
128. 
129. 
130. 

IS*- 
I32. 

133- 

*34- 
135- 
136- 
»37- 

138. 

I39. 
140. 
141. 
142. 

143- 

144. 

MS- 
146. 

H7. 
148. 


Human  Embryo  of  32  mm., 151 

Human  Embryo  of  34mm.     Front  View  of  Face, 152 

Human  Embryo  of  55  mm.     Seventy-five  Days, 152 

Human  Embryo  of  78  mm.     Three  Months, 153 

Front  View  of  the  Face  of  the  Embryo  Shown  in  Fig.  94,  .  • 153 

Human  Embryo  of  1 20  mm.      (?  One  Hundred  and  Ten  Days), 154 

Human  Embryo  of  n8mm.     One  Hundred  and  Six  Days, 154 

Human  Embryo  of  155  mm.     One  Hundred  and  Twenty-three  Days, 155 

Pig  Embryo  of  10  mm., 160 

Transverse  Section  of  Pig  Embryo  of  12  mm., facing  163 

Pig  Embryo  of  12.0  mm.     Reconstruction  from  the  Transverse  Sections,  Series  5  (drawn  by  Dr. 

F.  T.  Lewis], 163 

Anterior  Wall  of  the  Pharynx  of  a  Human  Embryo  of  3.2  mm.  (after  W.  His), 164 

Aortic  System  of  His's  Embryo  BJ,  4.25  mm.  (after  W.  His), 164 

Transverse  Section  of  Pig  Embryo  of  12   mm., facing  165 

Pig  Embryo  of  12.0  mm.     Reconstruction  from  the  Transverse  Sections,  Series  5  (drawn  by  Dr. 

F.  7\  Lewis], 165 

Transverse  Section  of  Pig  Embryo  of  1 2  mm., facing  167 

Pig  Embryo  of   12.0  mm.     Reconstruction  from  the  Transverse  Sections,  Series  5  (drawn  by  Dr. 

F.  T.  Lewis),     . .  167 

Transverse  Section  of  Pig  Embryo  of  12  mm., Jacin%  169 

Pig  Embryo  of  1 2.0  mm.     Reconstruction  from  the  Transverse  Sections,  Series  5  (drawn  by  Dr. 

F.  T.  Lewis) 169 

Pig  Embryo  of  15  mm., 170 

Fig  Embryo  of  20  mm., 172 

Reconstruction  of  a  Pig  Embryo  of  12.0  mm.  with  Indications  of  the  Planes  of  Sections  Figured,  .  174 

Pig,  I2.omm.     Transverse  Series  5.  Section  185, 176 

Portion  of  Fig.  113  More  Highly  Magnified, 179 

Pig,  12. o  mm.     Transverse  Series  5,  Section  198, 182 

Pig,  12. o  mm.     Transverse  Series  5,  Section  249, 185 

Pig,  12.0  mm.     Transverse  Series  5,  Section  292, i&& 

Pig,  12. o  mm.     Transverse  Series  5,  Section  321, 192 

Pig,  12. o  mm.     Transverse  Series  5,  Section  353, 194 

Pig,  12.0  mm.     Transverse  Series  5,  Section  470, 197 

Pig,  I2.omm.     Transverse  Series  5,  Section  513, 2OO 

Pig,  12. o  mm.     Transverse  Series  5,  Section  633, 203 

Pig,  12. o  mm.     Sagittal  Series  7,  Section  70, 206 

Pig,  I2.omm.     No.  7.     Sagittal  Section  25,      209 

Pig,  12.0  mm.     Frontal  Series  6,  Section  284, ' 212 

Pig,  12. o  mm.      Frontal  Series  6,  Section  340, 213 

Pig,  12.0  mm.      Frontal  Series  6,  Section  380, 214 

Pig,  12. o  mm.     Frontal  Series  6,  Section  572, 216 

Pig,  9.0  mm.     Transverse  Series  9,  Section  171, 218 

Pig,  9.0  mm.     Sagittal  Series  53,  Section  213, 220 

Pig,  9.0  mm.     Frontal  Series  54,  Section  194, 223 

Pig,  9.0  mm.     Frontal  Series  54,  Section  194, 224 

Pig,  9.0  mm.     Frontal  Series,  54,  Section  459, 228- 

Pig,  6.0  mm.     Transverse  Series  9,  Section  90, 229 

Pig,  6.0  mm.     Transverse  Series  9,  Section  519, 230- 

Pig,  17.0  mm.     Transverse  Series  51,  Section  464, 232 

Pig,  17.0  mm.     Transverse  Series  51,  Section  651, 236 

Pig,  17.0  mm.     Transverse  Series  51,  Section  759, 23& 

Pig,  17.0  mm.     Frontal  Series  39,  Section  63, 240 

Pig,  20.0  mm.     Transverse  Series  59,  Section  522, 241 

Pig,  20.0  mm.      Transverse  Series  59,  Section  522, .    .    .' 243 

Pig,  20.0  mm.      Transverse  Series  59,  Section  701, 245 

Pig,  20. o  mm.      Transverse  Series  59,  Section  1253, 24^ 

Pig,  20. o  mm.     Transverse  Series  59,  Section  1043, 249 

Pig,  20.0  mm.      Sagittal  Series  60,  Section   213, 252 

Pig,  20.0  mm.     Frontal  Section  of  Head.     Series  40,  Section  68, 254 

Pig,  2o.omm.      Frontal  Section  of  Head.     Series  40,  Section  84 256 

Pig,  20.0  mm.      Frontal  Section  of  Head.      Series  40,  Section  123, 257 


xvi  LIST  OF  ILLUSTRATIONS. 

FIG.  PAGE 

149.  Rabbit  Embryo  of  Thirteen  Days ;  Section  of  the  Eye, 260 

150.  Pig,  24.0  mm.     Transverse  Series  62,  Section  428, 261 

151.  Pig,  24.0  mm.     Sagittal  Series  63,  Section  30, 264 

152.  Embryo  Chick  with  about  Twenty-four  Segments.     Surface  View  from  the  Dorsal  Side, 272 

153.  Section  of  Chick  Embryo  with  about  Twenty-eight  Segments.     Transverse  Series  92,  Section  73,  .    .  275 

154.  Section  of  Chick  Embryo  with  about  Twenty-eight  Segments.     Transverse  Series  92,  Section  83,  .    .  275 

155.  Section  of  Chick  Embryo  with  about  Twenty-eight  Segments.      Transverse  Series  92,  Section  96,  .    .  276 

156.  Section  of  Chick  Embryo  with  about  Twenty-eight  Segments.     Transverse  Series  92,  Section  104,  .  277 

157.  Section  of  Chick  Embryo  with  about  Twenty-eight  Segments.     Transverse  Series  92,  Section  114,  .  279 

158.  Section  of  Chick  Embryo  with  about  Twenty-eight  Segments.     Transverse  Series  92,  Section  144,  .  281 

159.  Section  of  Chick  Embryo  with  about  Twenty-eight  Segments.     Transverse  Series  92,  Section  165,  .  284 

160.  Section  of  Chick  Embryo  with  about  Twenty-eight  Segments.     Transverse  Series  92,  Section  179,  .  285 

161.  Section  of  a  Chicken  Embryo  with  about  Twenty-eight  Segments.     Transverse  Series  92,  Section  220,  286 

162.  Section  of  a  Chicken  Embryo  with  about  Twenty-eight  Segments.     Transverse  Series  92,  Section  356,  287 

163.  Section  of  a  Chicken  Embryo  with  Twenty-eight  Segments.     Transverse  Series  92,  Section  419,  .    .    .  288 

164.  Chick  Embryo  with  about  Twenty-eight  Segments.     Transverse  Series  92,  Section  424, 289 

165.  Chick  Embryo  with  about  Twenty-eight  Segments.     Transverse  Series  92,  Section  427, 290 

166.  Horizontal  Section  of  a  Chick  Embryo  with  about  Twenty-eight  Segments, 291 

167.  Chicken    Embryo,  after  Twenty-seven   Hours'   Incubation,  with  Eight  Primitive   Segments  (after 

Duval), 296 

168.  Embryonic  Area  of  a  Rabbit  of  (?  Nine)  Days,  with  the  Placental  Area  Partly  Torn  off  (after  van 

Beneden  and  Julin), 298 

169.  Longitudinal  Section  of  a  Young  Chick  Embryo, 298 

170.  Chick  Embryo  with  Seven  Segments.     Transverse  Section  of  the  Head, 299 

171.  Chicken  Embryo  with  Seven  Segments.     Transverse  Section  through  the  Anlage  of  the  Heart,    .    ,  300 

172.  Chicken  Embryo,  Transverse  Section  across  the  Anlage  of  the  Heart  in  a  Stage  slightly  more  Ad- 

vanced than  Fig.  171, 301 

173.  Chicken  Embryo  with  Seven  Segments, 302 

174.  Chicken  Embryo  with  Seven  Segments.     Transverse  Section  across  the  Primitive  Groove,    ....  303 

175.  Blastodermic  Vesicle  of  a  Rabbit  at  Seven  Days, 309 

176.  Rabbit  Embryo  of  Seven  Days.     Transverse  Series  12,  Section  216,  through  the  Anterior  Portion 

of  the  Embryonic  Shield,       310 

177.  Rabbit  Embryo  of  Seven  Days.     Transverse  Series  15,  Section  39,  through  the  Posterior  Part  of 

the  Embryonic  Shield,     ....            ...                311 

178.  Ovum  of  White  Mouse,  with  the  First  Polar  Spindle  in  Tangential  Position  (after  J.  Sobotta),  .    .    .  313 

179.  Ovum  of  White  Mouse,  dividing  to  Produce  the  Polar  Globule  (after  J.  Sobotta), 314 

180.  Ovum  of  White  Mouse,  Showing  the  Metaphase  of  the  Division  Producing  the  First  Polar  Globule 

(after  J.  Sobotta}, 314 

181.  Ovum  of  White  Mouse,  after  the  Formation  of  the  Polar  Globule.     Both  Pro-nuclei  are  Present 

(after  J.  Sobotta), 315 

182.  Ovum  of  White  Mouse,  with  Two  Well-developed  Pro-nuclei  (after  J.  Sobotta), 315 

183.  Vertical  Section  of  a  Human  Uterus  of  the  First  Day  of  Menstruation, 318 

184.  Vertical  Section  of  a  Human  Uterus  (Decidua  vera),  One  Month  Pregnant, 323 

185.  Human  Uterus,  One  Month  Pregnant.     Section  of  Gland  from  the  Cavernous  Layer,  with  the  Epi- 

thelium Partly  Adherent  to  the  Walls, 323 

186.  Human  Uterus,  One  Month  Pregnant.     Section  of  a  Gland  from  the  Cavernous  Layer  with  the  Epi- 

thelium Loosened  from  the  Walls.     The  Epithelial  Cells  are  Swollen,       .                     324 

187.  Uterus  One  Month  Pregnant;    Portion  of   the  Compact  Layer  of   the  Decidua  Seen  in  Vertical 

Section, 325 

188.  Section  of  Human  Decidua  Reflexa  at  Two  Months, 326 

189.  Human  Uterus  about  Seven  Months  Pregnant.     Vertical  Section  of  the  Decidua  Vera  with  the 

Foetal  Membranes  in  situ, 328 

190.  Human  Placenta  in  situ,  about  Seven  Months.     Vertical  Section, 330 

191.  Human  Placental  Chorion  and  Amnion  of  the  Fifth  Month, 331 

192.  Human  Chorion  of  Seven  Months'  Placenta, 332 

193.  Adenoid  Tissue  from  a  Villus  of  a  Human  Placenta  of  Four  Months, 334 

194.  The  Human  Decidua  Serotina  at  Seven   Months.     The  Section  is  Taken  from  near  the  Margin  of 

the  Placenta, 335 

195.  Decidual  Cells  from  the  Section  Represented  in  Fig.  194, 336 

196.  Human  Placenta  at  Full  Term,  Doubly  Injected  to  show  the  Superficial  Distribution  of  the  Blood- 

vessels,   338 

197.  Human  Placenta  after  Delivery  at  Full  Term, 340 


LIST  OF  ILLUSTRATIONS.  xvii 

FIG.  PAGE 

198.  Section  of  a  Very  Young  Human  Chorion, 343 

199.  Portion  of  the  Preceding  Figure  More  Highly  Magnified, 344 

200.  Aborting  Villus  from  the  Human  Chorion  Lseve  of  the  Second  Month, 346 

201.  Fragment  of  the  Chorion  of  Fig.  69,  Highly  Magnified, 346 

202.  Isolated  Terminal  Branch  of  a  Villus  from  a  Human  Chorion  of  Twelve  Weeks, 347 

203.  Villous  Stem  from  a  Human  Placenta  of  the  Fifth   Month, 347 

204.  Terminal  Branches  of  a  Villus  from  a  Human  Placenta  at  Full  Term 347 

205.  Portion  of  an  Injected  Villus  from  a  Placenta  of  about  Five  Months, 347 

206.  Portion  of  a  Small  Injected  Villous  Stem  from  a  Placenta  of  about  Five  Months, 348 

207.  Transverse  Section  of  a  Human  Amnion  of  Two  Months, 349 

208.  Two  Sections  of  the  Human  Amnion, 350 

209.  Surface  View  of  the  Human  Amniotic  Epithelium  of  the  Fourth  Month, 350 

210.  Natural  Group  of  Nuclei  from  the  Mesoderm  of  the  Human  Amnion  of  the  Fifth  Month,    ....  351 

211.  Cross-section  of  a  Human  Umbilical  Cord  at  Term,  .    .            352 

212.  Section  of  the  Allantois  from  a  Human  Umbilical  Cord  of  Three  Months 352 

213.  Connective  Tissue  from  the  Umbilical  Cord  of  a  Human  Embryo  of  21  mm.     Stained  with  Alum 

Cochineal  and  Eosin, 352 

214.  Connective  Tissue  from  the  Umbilical  Cord  of  a  Human  Embryo  of  Three  Months,  Stained  with 

Alum  Cochineal  and  Eosin, 353 

215.  Ectoderm  of  an  Umbilical  Cord  of  a  Human  Embryo  of  Three  Months, 354 

216.  Section  of  the  Yolk-sac  of  a  Very  Young  Human  Embryo  (after  Fr.  Keibel'} 355 

217.  The  Precision  Microtome,  . .' 361 

218.  The  Automatic  Rotary  Microtome, 362 


ERRATA. 


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TEXT-BOOK  OF  EMBRYOLOGY. 


CHAPTER    I. 

GENERAL  CONCEPTIONS. 

The  student  of  embryology  should  start  with  as  clear  and  definite  a  concep- 
tion as  possible  of  what  he  is  to  gain  from  his  pursuit  of  that  science.  If  he  is  a 
student  of  biology  or  of  zoology,  he  must  appreciate  that  knowledge  of  the  laws 
of  development  is  an  indispensable  part  of  what  he  must  master  in  order  to 
understand  those  sciences.  He  must  appreciate  that  it  is  from  the  studies  of  the 
embryologist  that  are  derived  our  conceptions  of  the  nature  of  sex,  of  heredity,  of 
variation,  of  differentiation,  and  many  of  our  most  important  notions  concerning 
evolution,  both  of  the  individual  and  of  the  race.  He  will  learn,  further,  that  the 
embryo  illustrates  to  him  with  particular  clearness  the  fundamental  principles 
of  morphology.  If  he  be  a  medical  student,  he  will  find  in  embryology  first 
of  all  the  clue  to  the  intelligent  comprehension  of  the  anatomy  of  the  adult,  a 
comprehension  which  he  can  obtain  in  no  other  way,  but  he  will  also  gain  much 
knowledge  of  direct  practical  value  as  to  the  embryo  and  as  to  the  conditions  in 
the  adult,  acquaintance  with  which  is  invaluable  in  medical  practice.  And, 
finally,  he  will  find  that  it  throws  a  vast  light  on  pathology,  both  upon  the  prob- 
lems of  malformations  and  monstrosities,  and  also  upon  the  whole  question  of 
pathological  change  in  the  tissues. 

The  best  study  of  embryology,  therefore,  is  that  which  continually  passes 
beyond  the  direct  observations  to  the  conceptions  which  they  justify  and  which 
underlie  many  important  branches  of  science  which  are  related  to,  and  in  a  large 
part  dependent  upon,  embryology. 

The  student  ought  to  strive,  accordingly,  to  pass  from  the  direct  observation 
of  the  specimen  to  the  generalizations,  and  accustom  himself  to  regard  always 
each  special  preparation,  which  may  be  submitted  to  his  observation,  as  an  illus- 

2  I7 


18  GENERAL   CONCEPTIONS. 

tration  of  some  general  principle.  To  facilitate  his  reaching  this  result  the 
following  pages  are  arranged  so  as  to  offer  a  digest  of  some  of  the  more  important 
generalizations  and  fundamental  laws  of  embryology. 

The  Vertebrate  Type  of  Structure. 

When  one  traces  the  course  of  development  of  any  vertebrate,  one  finds, 
speaking  in  general  terms,  that  the  fundamental  characteristics,  which  are  more 
or  less  common  to  all  vertebrates,  are  those  which  first  appear.  Later,  there 
come  in  the  secondary  characteristics,  which  distinguish  one  class  from  another, 
and  still  later  the  subordinate  characteristics  by  which  the  smaller  subdivisions 
of  the  vertebrate  type  become  differentiated  one  from  another.  This  state- 
ment, however,  is  correct  only  if  we  add  to  it  certain  indispensable  limitations. 
Every  embryo  at  every  stage  of  its  development  is  an  individual  of  the  particular 
genus  and  species  to  which  it  belongs.  It  has  at  every  stage  peculiarities  which 
distinguish  it  from  every  other  species.  The  embryos  of  allied  forms  resemble 
one  another  more  closely  than  do  the  embryos  of  forms  which  are  only  distantly 
related  to  one  another.  The  specific  qualities  of  an  embryo  are,  however,  far 
more  difficult  to  recognize  than  those  of  the  adult,  and  the  student  will  be  far 
more  impressed  by  the  resemblances  between  embryos  than  by  their  differences. 
It  is  owing  to  this  very  fact  that  the  distinctive  peculiarities  of  the  species  are  not 
accentuated  in  the  embryo.  We  are  able  to  derive  from  the  embryos  themselves 
a  series  of  conceptions  which  render  it  comparatively  easy  to  perceive  the  domi- 
nant morphological  features  of  the  vertebrate  type. 

It  will  be  convenient  to  put  down  six  fundamental  characteristics  of  the  ver- 
tebrate type  as  the  most  important,  and  to  add  to  these  six  others  which  are 
also  fundamental,  but  perhaps  less  distinctive.  This  enumeration  is  necessarily 
arbitrary,  and  can  serve  only  to  facilitate  the  work  of  the  student.  When  his 
knowledge  deepens,  he  will  be  able  to  free  himself  from  the  limitations  which 
such  a  numerical  classification  has  put  on  his  understanding  of  the  matter. 

A.  The  six  most  important  characteristics  are : 

1.  The  pharynx  and  pharyngeal  structures  (gill  clefts,  nerves,  aortic 

arches,  heart). 

2.  The  notochord  or  structural  axis. 

3.  Hollow  nervous  system. 

4.  Limbs. 

5.  Position  of  mouth. 

6.  Division  of  the  coelom  into : 

(a)  segmented  part  (mesomeres) ; 

(6)  unsegmented  part   (splanchnocele),   which    is   subdivided    by 
the  septum  transversum. 


THE   VERTEBRATE  TYPE  OF  STRUCTURE.  19 

B.  Other  fundamental  but  less  distinctive  characteristics  are: 

7.  Stomach,  intestine,  and  mesentery. 

8.  Position  of  liver,  and  its  relation  to  veins. 

9.  Wolffian  tubules  and  ovotestis  (=  urogenital  ridge). 

10.  Urogenital  ducts  (Wolffian  and  Miillerian). 

1 1.  Special  sense-organs  (nose,  eye,  and  ear). 

12.  Hypophysis. 

The  pig  embryo  illustrates  all  these  characteristics,  and  we  shall  study  the 
ways  in  which  the  typical  mammalian  modifications  of  the  type  are  gradually 
evolved. 

Let  us  now  pass  in  review  these  twelve  characteristics : 

1 .  The  pharynx  is  the  cephalic  portion  of  the  digestive  canal,  and  it  acquires 
in  all  vertebrates  a  somewhat  complicated  structure.     This  complication  de- 
pends primarily  upon  a  series  of  lateral  outgrowths  from  the  pharynx  which  are 
known  by  the  name  of  gill  pouches.     They  are  symmetrically  arranged  and 
therefore  form  pairs.     They  are  designated  by  numbers,  the  pouch  which  lies 
nearest  to  the  mouth  being  called  the  first,  the  next  the  second,  and  so  on.     In 
many  of  the  lower  vertebrates  the  number  of  these  gill  pouches  varies  from  five 
to  perhaps  nine.     In  mammals  there  are  always  four  pairs  on  each  side.     In 
aquatic  vertebrates  the  pouches  acquire  each  an  opening  to  the  exterior  at  the 
side  of  the  neck,  and  are  then  designated  as  gill  clefts  or  branchial  clefts.     We 
find  that  the  position  of  the  clefts  determines  the  distribution  of  a  series  of  the 
most  important  of  the  cephalic  nerves  and  the  primitive  distribution  of  the 
branches  of  the  aorta  and  of  certain  important  muscles,  hence  the  morphological 
features  of  the  pharynx  have  a  profound  influence  upon  the  entire  anatomy  of 
the  body  in  that  region.     No  similar  pouches  are  formed  from  any  other  part  of 
the  digestive  canal. 

2 .  The  notochord  is  a  rod  of  cells  which  extends  nearly  the  entire  length  of 
the  embryo.     It  lies  in  the  median  plane,  a  little  below  the  ventral  edge  of  the 
central  nervous  system.     Its  cephalic  termination  is  always  in  the  neighborhood 
of  the  pituitary  body.     It  may  be  considered  the  primitive  structural  axis  of  the 
vertebrates.  There  are  vertebrates  in  which  it  is  the  only  structural  axis  ever  pro- 
duced, but  in  the  great  majority  of  vertebrates  there  is  developed  around  the 
notochord  a  series  of  skeletal  elements  which  we  know  as  vertebrae,  and  which 
make  a  new  structural  axis  in  these  forms.     The  notochord  in  these  animals  is 
found  to  run  through  the  bodies  of  the  vertebrae.     The  notochord  diminishes  in 
size  as  we  ascend  the  vertebrate  series.     It  is  of  very  considerable  diameter  in 
the  lowest  fishes,  smaller  in  amphibia  and  reptiles,  and  smallest  of  all  in  mam- 
mals.    Its  duration  through  the  life-history  of  the  individual  also  diminishes  as 


20  GENERAL   CONCEPTIONS. 

we  ascend  the  series,  for  we  find  that  in  the  lowest  fishes  it  persists  well  developed 
throughout  life ;  in  other  fishes  it  disappears  in  part,  in  amphibia  it  disappears 
almost  completely,  and  in  mammals  it  aborts  entirely,  and,  so  far  as  known,  no 
remnant  of  it  normally  persists  in  the  adult. 

3.  The  hollow  nervous  system.     This  is  found  in  vertebrates  only,  or  in 
animals  which  are  closely  related  to  vertebrates,  so  closely  that  to  many  natural- 
ists they  are  included  in  the  same  sub-kingdom.     The  hollow  nervous  system  is 
enlarged  in  the  region  of  the  head,  the  enlargement  constituting  the  brain.     The 
rest  of  it  is  of  smaller  size  and  constitutes  the  spinal  cord. 

4.  The  limbs.     There  are  two  pairs,  which  are  lateral  extensions  of  the  sur- 
face of  the  body  and  acquire  in  their  interior  a  skeleton  by  which  they  are  sup- 
ported and  muscles  by  which  they  are  moved.     No  homologous  structures  are 
known  in  any  invertebrate  animal. 

5.  The  position  of  the  mouth.     The  typical  invertebrate  mouth  is  sur- 
rounded by  the  nervous  system.     For  instance,  in  insects  or  in  the  jointed  worms 
(annelids)  there  is  a  brain,  so  called,  above  the  mouth,  and  a  strand  of  nervous 
tissue  running  down  on  either  side  of  the  body  past  the  mouth  to  join  the  gan- 
glion on  the  lower  side,  thus  completing  a  ring  of  nervous  material  through  which 
the  oesophagus  passes.     In  vertebrates,  on  the  other  hand,  the  mouth  is  not 
enclosed  by  any  oesophageal  ring,  and  the  entire  nervous  system  is  on  one  side  of 
the  body  and  dorsal  to  the  mouth. 

6.  The  division  of  the  primitive  body-cavity.     The  body-cavity  in  the  em- 
bryo is  known  by  the  comprehensive  name  of  the  coelom.     It  will  not  be  possible 
to  acquire  a  clear  idea  of  its  division  until  the  embryos  are  actually  studied.     It 
forms  many  parts.     Of  these,  there  are  two  series,  one  on  each  side  of  the  central 
nervous  system,  which  form  cavities  of  what  we  designate  as  the  primitive  seg- 
ments of  the  body.     There  are  also  two  large  divisions  which  extend  from  the 
region  of  the  head  to  that  of  the  future  pelvis,  one  division  for  each  side  of  the 
body.     These  two  large  parts  are  not  divided  into  segments  at  all,  though  the 
cavities  of  all  of  the  segments  are  primitively  connected  with  these  two  main 
divisions.     Comparatively  early  in  the  development  the  two  main  cavities  be- 
come connected  with  one  another  so  as  to  constitute  a  single  cavity  to  which  we 
apply  the  name  of  splanchnocele.     The  splanchnocele  surrounds  the  heart  of 
the  embryo,  where  we  recognize  it  as  the  pericardial  cavity,  and  it  extends 
through  the  future  abdominal  region,  where  we  recognize  it  as  the  abdominal 
cavity.     The  pericardial  and  abdominal  regions  of  the  cavity  are  separated  from 
one  another  in  the  embryo  by  a  broad  transverse  partition  which  bears  the  name 
of  septum  transversum.     This  septum  in  mammals  becomes  in  the  adult  the 
diaphragm.     It  is  one  of  the  most  striking  of  all  the  morphological  peculiarities 
by  which  vertebrates  are  distinguished  from  invertebrates. 


THE   VERTEBRATE  TYPE   OF  STRUCTURE.  21 

7.  The  stomach,  intestine,  and  mesentery.     The  division  of  the  digestive 
tract  of  vertebrates  into  these  two  fundamental  parts  is  very  characteristic. 
The  stomach  is  not  only  an  enlargement  of  the  digestive  canal,  but  also  may  be- 
distinguished  from  the  intestine  by  its  developing  glands,  which  are  specific  to  it 
and  unlike  those  of  the  intestine  proper.     The  mesentery  by  which  the  intestine 
is  suspended  to  the  dorsal  wall  of  the  abdomen  is  the  survival  of  the  original  parti- 
tion by  which  the  two  halves  of  the  splanchnocele  cavities  were  separated  from 
one  another.     The  cavities  in  the  abdominal  region  come  into  communication 
with  one  another  by  the  very  early  disappearance  of  the  partition  on  the  ventral 
side  of  the  intestine.     But  it  should  be  noted  at  once  that  a  portion  of  this  primi- 
tive ventral  partition,  or,  as  we  may  call  it,  ventral  mesentery,  persists  perma- 
nently in  relation  to  the  position  of  the  liver. 

8.  The  position  of  the  liver.     The  primitive  large  veins  of  the  embryo  pass 
through  the  septum  transversum,  and  it  is  in  connection  with  these  veins,  and 
as  an  appendage  to  the  septum  itself,  that  the  liver  is  developed. 

9.  The  urogenital  ridge.     Out  of  a  part  of  the  primitive  segments  there  are 
developed  excretory  organs,  and  these,  as  they  increase  in  size,  form  two  pro- 
tuberances on  the  dorsal  side  of  the  abdominal  cavity.      Each  protuberance  is 
what  we  know  as  the  urogenital  ridge,  so  named,  first,  on  account  of  its  form; 
and,  secondly,  on  account  of  its  producing  not  only  the  excretory  organs  proper, 
but  also  the  genital  glands. 

10.  The  urogenital  ducts.     There  is  primitively  a  single  duct  for  each  uro- 
genital ridge.     This  duct  is  commonly  known  as  the  Wolffian  duct.     A  little 
later  in  the  history  of  the  embryo  there  appears  a  second  duct  which  is  closely 
parallel  to  the  first,  but  which  has  no  connection  with  any  of  the  excretory  appa- 
ratus, and  is  destined  to  serve  later  as  the  female  genital  duct.     In  no  inverte- 
brate have  we  found  anything  homologous  with  these  two  ducts. 

11.  Special  sense-organs.     These  are  the  olfactory,  the  visual,  and  the  so- 
called  auditory  organs.     We  have  to  use  the  term  "  so-called  "  in  speaking  of  the 
auditory  organ  because  we  now  know  that  the  ear  in  the  lower  vertebrates  is  not 
an  organ  of  hearing,  but  an  organ  of  balancing  or  orientation,  and  it  is  only  in  the 
higher  vertebrates  that  there  is  added  to  this  primitive  function  that  of  audition 
proper.     It  seems  not  improbable  that  many  invertebrate  animals  have  sense- 
organs  which  are  homologous  with  those  of  vertebrates.     Nevertheless,  in  the 
vertebrate  type  there  are  many  peculiarities  which  are  distinctive,  and  these  we 
shall  best  learn  from  a  study  of  the  actual  development. 

12.  The  hypophysis.     The  hypophysis  is  the  embryological  name  applied 
to  the  structure  which  we  know  in  the  adult  as  the  anterior  lobe  of  the  pituitary 
body.     The  posterior  or  infundibular  lobe  is  a  portion  of  the  brain,  but  the  ante- 


22  GENERAL  CONCEPTIONS. 

rior  lobe  is  an  outgrowth  from  the  cavity  of  the  mouth  of  the  embryo.  Com- 
paratively early  in  the  development  of  the  individual  this  outgrowth  becomes 
entirely  separated  from  the  mouth-cavity  (from  the  walls  of  which  it  arose),  and 
forms  a  closed  vesicle.  It  exists  in  every  known  vertebrate  animal,  has  been 
much  studied,  but  still  remains  an  organ  the  significance  of  which  we  cannot  ex- 
plain. Its  absolute  persistency  and  the  uniformity  of  its  development  indicate 
that  it  is  an  organ  of  importance,  but  beyond  that  we  can  hardly  go. 

To  these  conceptions,  the  student  should  add  the  following  comprehensive 
morphological  notions :  The  mammalian  body  may  be  defined  as  two  tubes  of 
epithelium,  one  inside  the  other;  the  outer  tube  (epidermal  or  ectodermal)  is 
very  irregular  in  its  form;  the  inner  tube  (entodermal)  is  much  smaller  in  diam- 
eter, but  much  longer  than  the  outer  and  has  a  number  of  branches  (lung,  pan- 
creas, etc.),  and  is  placed  within  the  ectodermal  tube.  Between  these  two  tubes 
is  the  very  bulky  mesoderm,  which  is  divided  by  large  cavities  (abdominal  and 
thoracic)  into  two  main  layers,  one  of  which  is  closely  associated  with  the  epi- 
dermis and  forms  the  body- wall,  the  somatopleure  of  embryologists ;  the  other 
joins  with  the  entoderm  to  complete  the  walls  of  the  splanchnic  viscera,  and  con- 
stitutes the  splanchnopleure  of  embryologists.  The  mesoderm  is  permeated  by 
two  sets  of  cavities :  (i)  the  heart  and  blood-vessels ;  (2)  the  lymphatic  system. 
It  is  also  differentiated  into  numerous  tissues,  muscle,  tendon,  bone,  etc.,  and 
organs,  urogenital  system.  The  nervous  system,  although  developed  from  the 
ectoderm,  is  found  separated  from  its  site  of  origin,  and  completely  encased  in 
mesoderm. 

The  Principal  Modifications  of  the  Vertebrate  Type. 

Our  knowledge  of  human  development  being  at  the  present  time  very  incom- 
plete, it  is  often  necessary  to  supplement  that  knowledge  by  reference  to  facts 
of  observation  on  the  development  of  various  vertebrates.  Indeed,  the  best 
study  of  human  embryology  includes  more  or  less  comparative  work.  We  shall, 
therefore,  find  frequent  occasion  to  refer  to  the  development  of  many  vertebrate 
types.  Accordingly,  in  this  section  there  are  given  definitions  of  the  principal 
subdivisions  of  the  vertebrates  to  which  we  shall  have  occasion  to  refer. 

From  an  embryological  standpoint,  vertebrates  may  be  separated  into 
two  main  divisions,  which  are  commonly  designated  as  the  Amniota  and 
Anamniota,  distinguished  by  the  presence  or  absence  of  the  amnion,  the 
amnion  being  a  thin  membrane,  which  immediately  surrounds  the  embryo 
in  the  higher  forms.  It  occurs  in  reptiles,  birds,  and  mammals,  which  together 
constitute  the  Amniota.  It  is  absent  in  the  fishes  and  amphibians,  which 
therefore  constitute  the  Anamniota.  These  two  divisions  are  also  distin- 


PRINCIPAL  MODIFICATIONS  OF  THE   VERTEBRATE  TYPE.        23 

guished  by  other  peculiarities.  The  higher  forms  referred  to  all  have  the 
organ  known  as  the  allantois,  an  appendage  of  the  embryo,  which  is  lacking 
in  the  lower  forms.  The  comparative  anatomist  finds  many  points  of  re- 
semblance between  the  various  classes  of  fishes,  on  the  one  hand,  and  the 
amphibia,  on  the  other,  and  indicates  this  relationship  by  the  use  of  the  term 
Ichthyopsida,  which  means  "fish-like."  In  our  present  classification  the  term 
Ichthyopsida  is  synonymous  with  Anamniota.  The  comparative  anatomist 
further  recognizes  a  close  relationship  between  birds  and  reptiles,  and  puts 
these  together  under  the  common  designation  of  Sauropsida,  or  "  reptile-like." 

As  regards  the  fishes,  many  classifications  are  more  or  less  in  vogue  at  the 
present  time.  For  the  purposes  of  this  book,  the  following  names  for  the 
classes  have  been  adopted  as  names  generally  understood  and  sufficiently 
exact  to  meet  our  needs :  The  lowest  fishes  are  the  hag-fishes  and  lampreys, 
constituting  the  group  of  Marsipobranchs.  Next  comes  the  group  compris- 
ing the  sturgeon  and  its  allies,  for  which  we  have  retained  the  old  term  of 
Ganoids.  To  these  fishes  the  central  position  in  the  system  must  be  assigned, 
and  it  is  probable  that  the  higher  fishes  are  more  or  less  directly  descended 
from  Ganoid-like  forms.  They  fall  into  three  further  classes,  of  which  the 
largest  and  most  varied  is  that  of  the  bony  fishes,  or  Teleosts.  Another  class, 
known  as  the  Elasmobranchs,  comprises  the  sharks,  skates,  rays,  and  electric 
fishes.  The  last  class  is  known  as  the  Dipnoi,  or  lung  fishes,  which  comprise 
only  three  living  forms,  the  Ceratodus,  living  in  Australia,  the  Protopterus 
in  Africa,  and  the  Lepidosiren  in  South  America. 

The  amphibia  are  divided  into  two  classes,  the  Urodela,  of  which  the 
newts  and  salamanders  are  familiar  examples,  and  the  Anura,  of  which. the 
frogs  and  the  toads  are  the  best  known  representatives. 

As  to  the  reptiles,  it  is  unnecessary  to  consider  their  classification,  as  we 
shall  not  have  much  occasion  to  refer  to  them,  our  knowledge  of  their  em- 
bryology being  very  fragmentary  at  the  present  time,  save  for  a  rather  ex- 
tended series  of  observations  on  the  development  of  lizards.  As  regards  birds, 
it  may  be  noted  that  embryologists  have  worked  chiefly  upon  the  chick,  which 
has  been  for  a  century  the  classic  object  of  embryological  study.  There  are  com- 
paratively few  observations  on  the  development  in  other  species  of  birds. 

Mammals  are  divided  into  three  principal  classes.  Of  these,  the  lowest  is 
that  of  the  Monotremes,  of  which  the  only  living  representatives  are  found  in 
Australia  and  neighboring  islands,  a  very  few  species  concerning  the  develop- 
ment of  which  very  little  is  as  yet  known,  but  which  are  of  importance,  as  they 
resemble  in  certain  respects  the  reptiles  and  assist  us  in  drawing  comparisons 
between  the  reptilian  and  the  mammalian  types.  Of  this  class,  the  Australian 
duck-bill  may  be  mentioned  as  typical. 


24  GENERAL  CONCEPTIONS. 

The  second  class  is  that  of  the  Ma rsupials,  familiar  to  us  in  America  through 
the  common  opossum.  In  Australia  there  are  many  genera  and  species  of 
marsupials. 

Annelida 
Atriozoa 

Tunicata  (Ascidia) 
Cephalochorda 
Amphioxus 
Vertebrata 

Anamniota  (Anallantoidea) 
Ichthyopsida 
Pisces 

Marsipobranchia  (lampreys,  etc.) 
Ganoidea  (sturgeon,  etc.) 
Teleostea  (bony  fishes) 
Elasmobranchia  (sharks,  skates,  etc.) 
Dipnoi  (lung-fishes) 
Amphibia 

Urodela  (newts,  salamanders,  etc. ) 
Anoura  (frogs,  toads) 
Amniota  (Allantoidea) 
Sauropsida 

Reptilia  (lizards,  crocodiles,  snakes,  turtles,  etc.) 
Aves 
Mammalia 

Montotremata  (duck-bill,  etc.) 
Marsupialia  (opossum,  kangaroo,  etc.) 
Placentalia 

Unguiculate  series 

Insectivora  (moles,  etc. ) 
Cheiroptera  (bats) 

Rodentra  (rats,  rabbits,  guinea-pigs,  etc.) 
Carnivora  (cats,  dogs,  etc.) 
Primata  (lemurs,  monkeys,  apes,  man) 
Ungulate  series 

Ungulata  (horse,  sheep,  pigs,  etc.) 

The  third  class  comprises  the  majority  of  well-known  mammals,  and  may 
be  termed  the  Placentalia,  and,  for  embryological  purposes,  it  is  convenient  to 
consider  the  Placentalia  as  forming  two  principal  subclasses,  the  animals  with 
claws  and  the  animals  with  hoofs,  the  Unguiculates  and  the  Ungulates.  Of  the 


A  SUMMARY  OF  EMBRYOLOGICAL  DEVELOPMENT.  25 

Unguiculates,  we  shall  have  occasion  to  refer  to  the  Insecti-vora,  of  which  the  mole 
may  serve  as  a  type;  the  Cheiroptera,  or  bats;  the  Rodents,  including  the  rats, 
guinea-pigs,  rabbits,  etc.;  the  Carnivora,  cats,  dogs,  and  allied  animals;  and, 
finally,  the  Primates,  which  include  the  lemurs,  monkeys,  apes,  and  man. 

Of  the  Ungulates,  we  shall  have  occasion  to  refer  chiefly  to  the  pig  and  the 
sheep.  The  following  table  presents  these  animals  which  we  shall  have  occasion 
to  consider  in  their  proper  order. 

Of  the  invertebrate  animals  there  will  be  little  to  be  said.  There  are  two 
types  of  invertebrates  which  show  relationship  in  their  structure  to  true  verte- 
brates. One  of  these  is  the  class  of  jointed  worms,  or  Annelids;  the  other  is  the 
class  of  Atriozoa,  which  comprises  the  subdivisions  of  Tunicata  and  of  the  Ceph- 
alochorda.  All  of  our  observations  on  the  development  of  this  last  type  are  based 
on  the  one  genus,  Amphioxus,  which  will  therefore  be  the  name  which  we  shall 
use  whenever  we  have  to  refer  to  these  animals. 

Definition  of  Anlage. 

There  will  be  frequent  occasion  to  use  this  word  in  a  strictly  technical  sense- 
It  has  been  adopted  from  the  German,  as  there  is  no  satisfactory  English  equiva- 
lent for  it.  The  French  use  the  word  "ebauche,"  and  the  Italians  " abozzo." 
Attempts  have  been  made  to  introduce  Greek  derivatives  or  other  terms,  but 
they  have  not  met  with  success,  so  that  "Anlage  "  is  now  used  very  widely  both 
in  America  and  in  England.  It  may  be  defined  as  follows :  The  first  accumula- 
tion of  cells  in  the  developing  embryo  recognizable  as  the  commencement  of  a 
structure,  organs  or  part. 

A  Summary  of  Embryological  Development. 

The  following  summary  applies  to  what  is  known  of  vertebrates  only.  It 
would  require  some  modifications  to  be  applicable  to  the  whole  animal  kingdom. 
Each  individual  arises  from  a  single  cell  which  is  termed  the  impregnated  or  fer- 
tilized ovum.  From  this  all  embryological  study  starts.  The  fertilized  ovum 
has  its  earlier  history,  since  it  is  the  product  of  the  fusion  of  two  sexual  elements. 
It  is  a  living  cell,  and  therefore  contains  protoplasm  and  nucleus.  It  is  also 
furnished  with  a  certain  amount  of  material  known  as  yolk,  which  exists  in 
the  form  of  separate  granules  imbedded  in  the  protoplasm.  This  yolk  is  the 
reserve  food  material,  and  by  the  assimilation  thereof  the  protoplasm  of  the 
ovum  can  grow. 

The  first  step  in  development  is  the  repeated  division  of  the  original  cell  so 
that  there  is  produced  an  increasing  number  of  cells.  The  earlier  stages  of  this 
cell  multiplication  are  designated  as  the  segmentation  of  the  ovum.  This  name  is 


26  GENERAL   CONCEPTIONS. 

due  to  the  fact  that  the  process  was  first  observed  in  the  eggs  of  amphibia  in  the 
early  part  of  the  last  century,  before  the  cell  doctrine  had  been  established.  In 
default  of  a  better  name,  the  separate  cells  into  which  the  ovum  divided  were 
called  segments,  for  it  was,  of  course,  not  known  that  they  were  cells.  Although 
this  term  is  no  longer  appropriate,  it  is  still  universally  used  because  of  its  conve- 
nience. There  are  two  principal  types  of  ovum  known:  in  one  type  we  find 
only  a  small  amount  of  yolk  material;  in  the  other  a  very  large  amount.  There 
are  ova  known  intermediate  between  these  two  types.  When  the  ovum  is  of  the , 
first  type,  the  whole  of  it  undergoes  segmentation  at  once,  and  to  such  an  ovum 
the  term  holoblastic  is  applied.  In  the  second  type,  on  the  contrary,  we  find  that 
the  protoplasm  tends  to  accumulate  at  one  pole  of  the  cell  and  the  yolk  granules 
at  the  other.  The  protoplasmic  portion  exhibits  a  far  more  active  cell  division 
than  the  yolk-bearing  portion,  so  that  the  segmentation  seems  to  take  place 
exclusively  around  one  pole  or  part  of  the  ovum,  which  is,  therefore,  designated 
as  meroblastic.  After  the  segmentation  of  the  ovum  the  multiplication  of  the 
cells  continues,  and  they  gradually  arrange  themselves  in  such  a  manner  as  to 
form  three  distinct  sheets  or  laminae,  which  are  named  "germ-layers."  These 
layers  are  designated :  the  outermost  as  Ectoderm,  the  innermost  as  Entoderm, 
and  the  middle  as  Mesoderm.*  From  an  embryological  point  of  view  the  im- 
—porta'nce^of  these  three  primitive  germ-layers  cannot  be  over-emphasized.  The 
principal  occupation  of  the  student  will  be  tp  familiarize  himself  with  the  appear- 
ance of  these  Jayers  and  the  modifications  which  they  undergo,  and  the  adult 
tissues  which  are  produced  from  them.  They  dominate  every  phase  of  develop- 
ment, the  form  of  every  organ,  the  production  of  every  tissue>  Their  impor- 
tance is  so  great  that  embryology  might  almost  be  defined  as  the  science  of 
germ-layers. 

The  primitive  germ-layers  consist  of  very  simple  cells,  and  are  themselves 
at  first  extremely  simple  in  their  organization.  The  majority  of  the  cells  which 
they  contain  undergo  a  greater  or  less  degree  of  modification  as  development 
progresses.  This  modification  is  termed  differentiation,  and  is  more  fully  con- 
sidered in  our  next  section,  on  Cytomorphosis.  It  is  probable,  however,  that  a 
certain  number  of  the  cells  very  early  in  the  development  are  set  apart,  preserv- 
ing the  primitive  character  of  their  protoplasm  and'taking  no  share  in  the  forma- 
tion of  the  tissues  of  the  body.  These  cells,  comparatively  unmodified,  are 
known  as  the  germ-cells.  Their  significance  is  more  fully  explained  in  the  section 
on  Heredity.  As  the  remaining  cells  form  part  of  the  body  of  the  individual, 

*  Some  English  and  occasionally  Continental  authors  use  other  terms  for  the  germ-layers  :  namely,  for 
ectoderm,  eniblast;  for  entoderm,  hypoblast;  for  mesoderm,  mesoblast.  I  have  preferred  to  maintain  the  older 
terms  which  have  been  in  almost  universal  use  for  a  century. 


CYTOMORPHOSIS.  -  27 

they  may  be  designated  as  somatic  cells.  Besides  the  process  of  differentiation 
of  the  cells,  we  find  that  the  production  of  organs  is  largely  dependent  upon  the 
unequal  growth  of  the  germ-layers,  one  part  growing  rapidly,  another  more 
slowly,  so  that  the  layers  acquire,  as  the  embryo  develops,  a  more  or  less  com- 
plicated form,  owing  to  the  folding  of  the  layers.  The  general  principles  which 
govern  these  important  developments  are  considered  in  the  section  upon  the 
Relations  of  Surface  to  Mass. 

Cytomorphosis. 

This  term  is  used  to  designate  comprehensively  all  the  structural  modifica- 
tions which  cells  or  successive  generations  of  cells  may  undergo,  from  the  earliest 
undifferentiated  stage  to  their  final  destruction.  It  will  be  convenient,  though 
somewhat  arbitrary,  to  distinguish  four  fundamental  successive  stages  of  cyto- 
morphosis.  These  stages  are  (i)  the  undifferentiated  stage;  (2)  the  stage  of 
progressive  differentiation,  which  itself  often  comprises  many  successive 
stages;  (3)  the  regressive  stage  or  that  during  which  degeneration  or  necro- 
biosis  occurs;  (4)  the  stage  of  the  removal  ofathe  dead  material. 

In  the  various  parts  of  the  body  we  find  these  stages  to  succeed  one  another 
at  varying  rates,  and  there  are  always  to  be  found  in  every  living  body  a  consid- 
erable number  of  cells  which  have  only  passed  through  a  certain  differentiation 
and  do  not  present  any  of  the  phenomena  of  degeneration  or  of  death.  On  the 
other  hand,  there  are  cells  at  every  epoch  of  life  after  the  embryonic  period 
which  degenerate  and  die  off,  although  the  life  of  the  individual  is  uninterrupted. 
At  any  given  moment  the  body  consists  of  cells,  which  have  made  unequal 
progress  through  the  cytomorphic  cycle. 

i.  The  Undifferentiated  Stage. — A  fertilized  ovum  is  an  undifferentiated 
being,  although  it  has  a  very  complex  organization.  As  it  has  only  one  nucleus, 
there  can  be  no  variety  of  nuclei.  The  term  "undifferentiated  "  therefore  ap- 
plies especially  to  the  protoplasm  which  seems  to  have  a  uniform  essential 
structure  throughout,  although  the  masses  and  strands  of  protoplasm  may 
exhibit  characteristic  peculiarities,  especially  in  relation  to  the  distribution  of 
the  yolk.  In  the  adult  tissues,  on  the  contrary,  we  see  that  the  protoplasm  of 
the  cells  of  different  kinds  offers  many  varieties  of  structure  visible  with  the 
microscope.  We  may  legitimately  conclude  that  the  absence  of  similar  visible 
peculiarities  in  the  egg,  by  which  one  part  may  be  distinguished  from  another,  is 
evidence  of  uniformity  of  structure  throughout  the  egg.  We  have  also  direct 
experimental  proof  that  the  egg  is  uniform  throughout,  or,  to  use  a  better  phrase, 
that  the  egg  is  isotropic.  Pfliiger,  in  1884,  proved  that  the  side  of  the  frog's  egg 
which  normally  develops  into  the  ventral  surface  of  the  embryo  can  be  made  to 


28  GENERAL   CONCEPTIONS. 

develop  into  a  perfectly  typical  dorsal  surface.  The  frog's  egg  has  a  small  white 
area,  which  normally  lies  underneath,  the  larger,  darkly  pigmented  area  of  the 
egg  alone  showing  from  above.  Out  of  the  dark  area  the  back,  with  the  nervous 
system  and  other  parts,  takes  its  origin.  If  the  eggs,  freshly  fertilized,  are 
fastened  with  the  white  side  up,  then  the  white  side  produces  an  absolutely 
normal  back  and  nervous  system,  normal  as  to  form  and  function,  though  lack- 
ing the  typical  pigmentation.  These  observations  were  confirmed  by  Born,  who 
further  discovered  that  the  segmentation  nucleus  always  rises  toward  the  upper 
side  of  the  egg,  and  that  the  position  of  the  nucleus  determines  which  part  of 
the  ovum  shall  become  the  dorsal  side  of  the  embryo.  Another  set  of  experi- 
ments by  Oskar  Schultze  demonstrated  that  both  the  unpigmented  and  the 
pigmented  sides  of  the  same  egg  could  be  made  to  produce  dorsal  structures. 
Another  class  of  experiments,  which  were  first  made  by  Hans  Driesch,  have 
revealed  that  the  earliest  cells  (segmentation  spheres,  blastomeres,  or  cleavage 
cells,  as  they  are  variously  called)  produced  by  the  ovum  preserve  the  undiffer- 
entiated  qualities  of  the  parent  egg,  and  may  develop  in  one  way  or  another 
according  to  circumstances.  The  egg  of  a  sea-urchin  divides  into  two  cells, 
each  of  which  multiplies  and  normally  gives  rise  to  half  of  the  body  of  the  animal. 
By  somewhat  violent  shaking  the  two  cells  may  be  artificially  separated;  each 
cell  may  then  develop  into  a  complete  larval  sea-urchin,  but  of  half  the  normal 
size  only.  Similar  experiments  have  since  been  made  by  several  investigators, 
who  have  obtained  like  results  with  other  animals,  vertebrate  as  well  as  inver- 
tebrate. Even  more  remarkable  larvae  have  been  raised  from  blastomeres  of  the 
four-cell  and  eight-cell  stages  of  segmentation,  producing  larvae  of  one-fourth  and 
one-eighth  the  normal  size.  Zoja  claims  to  have  repeated  the  experiment  suc- 
cessfully on  the  eggs  of  Clytia,  and  to  have  obtained  one-sixteenth  larvae. 

The  facts  offered  suffice  to  illustrate  the  two  aspects  of  our  conception  of  the 
undifferentiated  condition  of  living  matter.  The  first  aspect  is  morphological 
and  presents  to  us  the  apparent  uniformity  of  the  visible  minute  structure  of 
protoplasm.  While  we  readily  admit  that  the  uniformity  may  be  only  apparent 
in  the  sense  that  we  fail  to  observe  fine  differences,  yet  we  none  the  less  maintain 
that  the  uniformity  is  real,  because  there  is  an  absence  of  variations  of  structure 
comparable  to  the  variations  which  we  can  observe  in  the  cells  of  adult  tissues. 
The  second  aspect  is  physiological,  and  offers  to  our  view  the  wide  range  of  possi- 
bilities in  the  future  developmental  history  and  growth  of  the  protoplasm.  The 
fate  of  the  protoplasm  of  any  given  part  of  the  ovum  is  not  fixed ;  but  if  its  con- 
ditions of  development  are  changed,  its  fate  is  changed.  A  few  years  ago  the 
mosaic  hypothesis  was  advanced  by  W.  Roux,  and  it  has  been  vigorously  de- 
fended by  him.  According  to  the  mosaic  theory,  the  egg  is  a  mosaic  pattern, 


CYTOMORPHOSIS.  29 

each  member  of  which  has  its  predestined  history.  It  is  fortunate  for  our  com- 
prehension of  embryological  processes  that  we  are  already  able  to  say  that  Roux's 
hypothesis  is  erroneous. 

We  must  start,  then,  with  the  right  conception  of  the  ovum,  every  part  of 
the  protoplasm  of  which  is  to  be  regarded  as  potentially  capable  of  producing 
any  or  all  of  the  tissues  of  the  adult. 

-2.  Differentiation. — This  may  be  defined  as  a  process  by  which  the  structure 
of  the  cells  is  modified,  so  that  cells  become  dissimilar  in  structure  by  acquiring 
an  organization  which  adapts  them  to  special  functions.  The  cells  which  arise 
during  the  segmentation  of  the  ovum  differ  but  slightly  from  one  another.  As 
development  progresses  we  find  the  cells  change,  some  in  one  way,  some  in  an- 
other, so  that  many  kinds  of  cells  are  produced,  but  of  each  kind  we  find  a  large 
number  of  cells.  Each  kind  of  cell  may  be  said,  roughly  speaking,  to  form  a 
tissue  for  itself.  Cells  of  each  tissue  offer  visible  peculiarities  by  which  they  may 
be  readily  distinguished  from  one  another  under  the  microscope.  It  thus  ap- 
pears that  the  production  of  tissues  is  the  main  result  of  differentiation,  so  that 
this  process  of  development  may  be  fairly  accurately  defined  as  equivalent  to 
histogenesis.  As  to  the  factors  which  cause  differentiation,  we  have  no  satis- 
factory knowledge.  We  can,  at  present,  only  note  the  changes,  when  they 
acquire  such  magnitude  as  to  become  microscopically  visible.  As  to  the  phys- 
iological conditions  which  cause  these  changes  we  have  almost  no  conceptions. 
It  is  probable  that  the  nucleus  has  a  leading  role  to  play,  but  our  knowledge  of 
this  role  is  too  little  advanced  to  permit  a  profitable  discussion  of  the  subject 
here. 

The  actual  process  of  differentiation  shows  itself  both  in  the  protoplasm 
and  in  the  nucleus  of  the  cell.  The  changes  in  the  latter  are  the  more  conspicu- 
ous, and  therefore  the  better  known.  The  changes  in  the  nucleus  have  still  to 
be  adequately  studied.  The  changes  in  the  protoplasm  are  twofold:  First,  in 
the  intimate  structure  of  the  protoplasm  itself  and  in  the  size  and  disposition 
of  its  strands  and  filaments ;  secondly,  in  the  character  of  the  various  substances 
to  be  found  imbedded  in  the  protoplasm.  These  two  kinds  of  change  are  well 
illustrated,  the  first,  by  the  nerve-cells;  the  second,  by  the  gland  cells,  for  in- 
stance, in  the  pancreas.  The  student  can  easily  see  that  the  character  of  the 
protoplasm  in  the  adult  nerve-cell  differs  profoundly  from  that  of  a  cell  from  one 
of  the  embryonic  germ-layers,  and  that  the  body  of  the  nerve-cell  consists  of 
protoplasm  with  little,  if  any,  of  other  substances  imbedded  in  it.  In  the  se- 
cretory cells  of  the  pancreas  the  zymogen  granules  are  conspicuous;  their  dis- 
tribution, uniform  size,  and  refractile  qualities  demonstrate  immediately  their 
unlikeness  to  anything  found  in  the  embryonic  cells.  These  granules  are  not 


30  GENERAL   CONCEPTIONS. 

protoplasm,  but  particles  imbedded  in  the  protoplasm  or,  as  they  may  be  called, 
enclosures. 

The  Law  of  Genetic  Restriction. — Another  fundamental  idea,  which  it  is 
most  important  for  the  student  to  grasp,  is  that  differentiation  acts  as  a  pro- 
gressive restriction  upon  the  further  development.  Each  successive  stage  of 
differentiation  puts  a  narrower  limitation  upon  the  possibility  of  further  advance. 

The  range  of  possible  changes  at  any  given  time  is  determined  not  merely 
by  the  nature  or  kind,  but  also  by  the  stage  or  degree  of  the  previous  differentia- 
tion. The  law  of  genetic  restriction  dominates  the  entire  ontogeny.  In  order  to 
illustrate  it  and  to  emphasize  it,  it  will  be  profitable  to  consider  a  few  illustra- 
tions from  each  of  the  germ-layers.  First,  then,  the  ectoderm.  This  layer  early 
separates  into  two  parts,  one  to  form  the  nervous  system,  the  second  the  epider- 
mis; the  nervous  part  thereafter  never  forms  epidermal  structures,  the  epider- 
mal part  never  forms  a  medullary  canal.  The  central  nervous  system  retains  in 
part  a  simple  epithelial  character  (ependyma  proper),  but  most  of  its  walls 
become  nervous  tissue;  its  cells  pass  from  the  indifferent  stage  and  become  neu- 
roglia  cells  or  young  nerve-cells  (neuroblasts).  Neuroglia  cells  never  become 
anything  else,  and  the  nerve-cells  are  always  nerve-cells  to  the  end.  Next,  as 
to  the  entoderm.  Wherever  in  it  specialization  takes  place,  as  in  the  tonsil, 
thymus,  thyroid,  oesophagus,  liver,  or  pancreas,  each  territory  of  cells  keeps 
its  characteristics  and  never  assumes  those  of  another  territory.  Finally,  as 
to  the  mesoderm.  It  is  found  very  early  to  include  in  vertebrate  embryos 
four  kinds  of  cells,  of  which  the  most  numerous  are  the  undifferentiated  cells, 
the  other  three  kinds  being  the  endothelium  of  blood-vessels,  red  blood-cells) 
and  germ-cells.  All  of  these  are  precociously  specialized;  they  are  few  in 
number,  yet  they  are  probably  the  parents  of  all  the  cells  which  are  produced  of 
their  kind  each  throughout  life.  Passing  on  to  a  later  stage,  we  note  that 
when  a  striated  muscle-fiber  is  produced  a  striated  muscle-fiber  it  always  re- 
mains, and  it  never  becomes  anything  else. 

Two  Types  of  Differentiation. — There  are  two  distinct  types  of  cell  differen- 
tiation which  I  think  have  not  hitherto  .been  clearly  recognized  or  defined.  For 
both  types  the  starting-point  is  the  same — the  undifferentiated  embryonic  cell. 
In  one  type  we  find  that,  as  the  cells  proliferate,  a  portion  of  them  only  under- 
goes differentiation,  and  another  portion  remains  more  or  less  undifferentiated 
and  retains  more  or  less  fully  the  power  of  continued  proliferation.  The  epi- 
dermis is  a  good  representative  of  this  type.  Its  basal  layer  consists  of  embryo- 
nic cells,  which  multiply;  some  of  these  cells  move  into  the  upper  layers,  enlarge, 
and  differentiate  themselves  into  horny  cells;  others  remain  in  the  basal  layer 
and  continue  to  multiply.  The  progeny  of  a  given  basal  epidermal  cell  do  not 


CYTOMORPHOSIS.  31 

all  have  the  same  fate,  but  divide  themselves  into  two  kinds  of  cells,  one  kind 
retaining  the  ancestral  character,  the  other  becoming  something  new  and  unlike 
the  parent  cell.  Differentiation  according  to  the  second  type  is  characterized 
by  its  inclusion  of  all  the  cells.  This  type  has  its  culminating  and  most  perfect 
illustration  in  the  central  nervous  system,  where  comparatively  early  in  em- 
bryonic life  all  the  cells  become  specialized,  and  with  the  acquisition  of  specializa- 
tion they  forfeit  their  power  of  multiplication — the  neuroglia  cells  partly,  the 
nerve-cells.wholly.*  The  growth  of  the  brain  after  early  stages  depends  not  on 
the  proliferation  of  cells,  but  chiefly  upon  the  increase  in  size  of  the  individual 
cell.  The  correctness  of  this  statement  is  not  affected,  in  my  belief,  by  the  fact 
that  epithelial  portions  of  the  medullary  tube  in  comparatively  late  stages  may 
be  added  to  the  nervous  portion,  the  cells  multiplying  rapidly,  as  we  see  at  the 
growing  edge  of  the  young  cerebellum.  The  brain  here  grows  by  the  addition  of 
cells  in  the  indifferent  stage,  but  as  soon  as  these  cells  are  differentiated  they 
conform  to  the  general  law  and  divide  no  more  (neurones)  or  slowly  (glia  cells). 

The  importance  to  pathologists  of  a  thorough  knowledge  of  the  genesis  of 
the  tissues  from  their  germ-layers  can  hardly  be  emphasized  too  strongly,  for  it  is 
more  than  probable  that  all  pathological  tissues  are  as  strictly  governed  by  the 
law  of  genetic  restriction  as  are  the  normal  tissues. 

3.  Regression. — The  use  of  this  term  does  not  imply  that  a  cell  can  move 
backward  after  differentiation  into  a  stage  of  lower  differentiation  or  into  an 
undifferentiated  condition.  So  far  as  we  know  at  present,  such  a  change  does 
not  occur,  and  we  therefore  look  upon  it  as  impossible.  Regressive  changes  are 
very  unlike  the  constructive  changes  which  appear  in  differentiation,  for  they  are 
destructive.  They  fall  into  three  main  groups:  first,  changes  of  direct  cell 
death;  second,  necrobiosis  or  indirect  cell  death  preceded  by  changes  in  cell 
structure;  third,  hypertrophic  degeneration  or  indirect  cell  death  preceded  by 
growth  and  structural  change  of  the  cell,  often  with  nuclear  proliferation.  Di- 
rect cell  death  implies  that  the  cell  loses  its  vitality,  and,  being  dead,  disin- 
tegrates; or,  may  be,  is  removed  by  some  means,  chemical  or  phagocytic,  before 
disintegration  occurs.  Necrobiosis  and  hypertrophic  degeneration  are  normal 
processes,  which  invariably  occur  in  the  normal  body  and  play  an  important  role 
in  its  development.  Without  their  occurrence  on  a  large  scale  the  normal  round 
of  human  life  would  be  impossible.  The  student  should  free  himself  from  the 
unfortunate  tradition  that  these  processes  are  exclusively  pathological. 

Correct  notions  on  this  subject  are  so  important  that  a  few  illustrations  may 
be  mentioned.  Let  us  begin  with  necrobiosis.  There  are  organs  whose  exist- 

*With  possibly  very  rare  exceptions. 


32  GENERAL   CONCEPTIONS. 

ence  is  limited  in  time,  such  as  the  thymus  and  foetal  kidney.  These  organs 
attain  their  full  differentiation,  their  elements  during  the  next  stage  die  off  and 
finally  are  resorbed,  most  of  the  organ  disappearing.  Another  familiar  illustra- 
tion is  offered  by  the  notochord,  which  in  mammals  totally  disappears.  Cell 
death  on  a  large  scale  is  a  common  phenomenon  of  the  tissues.  It  occurs  in  the 
cartilage  both  when  the  cartilage  is  permanent  and,  even  more  conspicuously, 
when  the  cartilage  gives  way  to  bone,  the  disintegration  of  the  cartilage  cells 
preceding  the  irruption  of  the  bone-forming  tissues.  It  occurs  among  the  gland 
cells  of  the  intestine,  in  the  pregnant  uterus,  and  in  all  the  tissues  of  human 
decidua  reflexa.  Degeneration  in  the  stricter  sense  of  the  ante-mortem  and 
hypertrophic  change  of  cell  structure  is  also  of  wide-spread  occurrence  in  the 
healthy  body.  Perhaps  no  instance  of  this  is  more  familiar  than  the  production 
of  horny  tissue  in  the  epidermis  or  elsewhere.  That  fatty  degeneration  takes 
place  normally  has  long  been  taught,  while  mucoid  and  colloid  degeneration  are 
so  obviously  normal  that  we  commonly  think  of  their  pathological  occurrence  as 
merely  an  exaggeration  of  a  normal  state.  Hypertrophic  degeneration  is  an 
extremely  common  pathological  process,  but  it  also  occurs  as  a  normal  process, 
as,  for  example,  in  epidermal  cornification,  as  just  mentioned,  and  very  strik- 
ingly in  the  production  of  giant-cells  (myeloplaxes,  etc.),  and  on  an  astounding 
scale  in  the  uterine  tissues  during  pregnancy  in  many,  perhaps  all,  mammals. 

4.  The  Removal  of  Cells. — The  sloughing  off  of  cells  is  one  of  the  most  fam- 
iliar phenomena,  since  it  occurs  incessantly  over  the  epidermis  and  with  hairs. 
Its  part  in  menstruation  and  its  colossal  role  in  the  after-birth  are  known  to  all, 
and  every  practitioner  is  accustomed  to  look  for  shed  cells  in  urinary  sediment. 
Large  numbers  of  cells  are  lost  by  the  intestinal  epithelium.  The  destruction  of 
blood-corpuscles  is  incessant,  and  we  might  greatly  extend  the  list  of  these  illus- 
trations. Owing  to  the  enormous  loss  of  cells  to  which  the  body  is  subject,  there 
is  provision  to  make  good  this  loss.  This  provision  is  called  "regeneration," 
and  has  been  dealt  with  in  an  enormous  number  of  investigations.  During  em- 
bryonic life  regeneration  plays  a  comparatively  insignificant  part,  and  we  shall 
not  have  to  deal  with  it  further. 

Of  the  four  stages  of  cytomorphosis,  the  second,  or  stage  of  differentiation, 
is  that  which  will  principally  claim  our  attention.  But  we  cannot  fully  under- 
stand the  developmental  processes  unless  we  also  have  constantly  in  mind  the 
normal  degeneration  and  death  of  cells,  even  in  the  embryo. 

Comparison  of  Larval  and  Embryonic  Types  of  Development. 

We  have  seen  in  the  preceding  section  that  the  first  cells  produced  in  devel- 
opment from  the  ovum  are  undifferentiated,  and  are  capable  of  development  in 


LARVAL  AND  EMBRYONIC  TYPES  OE  DEVELOPMENT.  33 

many  and  varied  directions.  The  more  they  become  specialized,  the  more  their 
possibilities  of  further  varied  development  are  decreased.  It  is  thus  obvious 
that  the  greater  the  number  of  cells  of  the  undifferentiated  type  that  can  be  pro- 
duced, the  greater  will  be  the  number  of  elements  which  can  be  later  differen- 
tiated. Hence,  the  more  the  period  for  the  production  of  undifferentiated  cells 
is  prolonged  and  the  commencement  of  differentiation  postponed,  the  more 
complex  may  be  the  degree  of  organization  ultimately  attainable. 

It  is  convenient  to  designate  the  undifferentiated  cells  as  they  asire  from 
the  segmentation  of  the  ovum  by  the  term  "  embryonic  cells."  The  object  of  this 
section  is  to  point  out  that  the  larval  type  of  development  is  less  favorable  for 
the  multiplication  of  embryonic  cells  than  is  the  embryonic  type;  and,  further, 
that  the  embryonic  type  becomes  more  and  more  marked  as  we  ascend  in  the 
animal  kingdom. 

The  Larval  Type. — In  the  lower  multicellular  animals  we  encounter  only 
larvae;  sponges,  jellyfish,  starfish,  and  worms  all  pass  through  their  early  stages 
as  larvae.  Now,  larvae  are  animal  forms  which  have  to  obtain  their  own  food 
and  to  protect  themselves  against  enemies.  They  are,  therefore,  provided  with 
a  variety  of  organs,  or,  as  we  may  say,  with  differentiated  tissues  which  enable 
them  to  perform  the  various  physiological  functions  which  are  necessary  for  the 
maintenance  of  their  existence.  The  differentiation  of  tissues  comes  in  very 
early. 

The  Embryonic  Type. — True  embryos  arise  from  eggs  which  contain  a  more 
or  less  considerable  amount  of  yolk  or  nutritive  material,  the  presence  of  which 
renders  unnecessary  any  activity  on  the  part  of  the  embryo  to  obtain  its  food- 
supply;  and  we  find,  moreover,  that  these  embryos  are  protected  by  hard  shells 
and  other  devices  from  their  enemies.  Their  only  task  is  to  pursue  their  own 
development.  Under  these  circumstances  it  is  possible  for  the  embryos  to  con- 
tinue for  a  long  time  the  production  of  embryonic  cells,  and  we  observe  that  the 
beginning  of  the  differentiation  proper  is  correspondingly  postponed.  The 
transition  from  the  larval  to  the  embryonic  type  is  very  gradual.  The  yolk 
appears  in  the  lower  animals  in  small  quantities,  increasing  in  some  of  the  higher 
types  and  attaining  its  maximum  in  some  of  the  highest.  Since  the  embryo  is 
dependent  on  the  yolk,  and  since  the  yolk  exists  only  in  the  higher  forms  in  suffi- 
cient quantities,  it  follows  that  fully  typical  embryos  can  occur  exclusively  in  the 
higher  animal  types. 

In  the  mammalia  the  ovum  contains  a  rather  small  quantity  of  yolk,  yet 
the  mammals  are  the  highest  animals  and  develop  most  perfectly  according  to 
the  embryonic  type.  This  peculiarity  is  due  to  the  fact  that  two  special  physi- 
ological devices  have  been  evolved  in  the  mammals  to  supply  food  to  the  devel- 


34  GENERAL  CONCEPTIONS. 

oping  embryo.  First,  there  is  a  special  relation  established  between  the  embryo 
and  the  uterus  by  means  of  a  complicated  adjustment  of  embryonic  and  uterine 
tissues,  which  supplies  nutrition  to  the  embryo  from  the  blood  of  the  mother. 
Second,  there  are  the  mammary  glands,  which  also  serve  the  same  function.  By 
these  two  devices  the  embryo  is  even  more  completely  freed  from  the  necessity 
of  seeking  its  food  and  protecting  itself  than  is  the  case  with  those  forms,  such  as 
the  birds  or  elasmobranchs,  in  which  the  supply  of  food  material  is  very  large. 

Germ=layers. 

The  germ-layers  are  the  firk  groups  of  cells  to  arise  as  the  result  of  the  seg- 
mentation of  the  ovum.  They  are  three  in  number,  and  each  forms  a  distinct 
sheet  or  lamina.  As  stated  on  page  26,  these  three  primitive  layers  are  termed 
"ectoderm,"  "mesoderm,"  and  "entoderm."  The  ectoderm  is  the  most  exter- 
nal of  the  three,  and  upon  the  outside  of  the  body  parts  of  the  ectoderm  remain 
permanently  to  constitute  the  outside  skin  or  epidermis.  From  its  very  position 
it  necessarily  is  the  part  of  the  body  to  come  into  relation  with  the  external 
world,  and  accordingly  we  find  that  its  two  great  duties  are  to  produce  the  pro- 
tective covering  of  the  body  and  the  apparatus  for  receiving  and  utilizing  sensa- 
tions; in  other  words,  the  chief  sensory  organs  and  the  nervous  system.  The 
entoderm,  on  the  contrary,  forms  the  internal  cavity  of  the  digestive  canal  and 
its  appendages.  It  therefore  is  concerned  chiefly  with  the  production  of  the  or- 
gans of  digestion,  and  appears  in  the  adult  as  the  epithelium  of  the  digestive  and 
respiratory  organs  and  of  the  glands  appended  to  the  digestive  tract.  The 
mesoderm,  lying  as  it  does  between  the  other  two  layers,  is  shut  off  by  them 
from  direct  relation  with  the  external  world  or  with  food-matter,  and  is  accord- 
ingly restricted  to  a  series  of  internal  functions,  of  which  four  are  especially 
important:  the  function  of  movement,  of  supporting  the  body,  especially  the 
parts  produced  from  the  ectoderm  and  entoderm,  of  circulation,  either  of  blood 
or  of  lymph  through  definite  channels,  and,  finally,  of  excretion.  It  is  from  the 
middle  germ-layer,  therefore,  that  the  muscular  tissues  arise,  that  the  connective 
and  skeletal  tissues  arise,  that  the  blood,  blood-vessels,  and  lymphatics  arise, 
and  that  the  excretory  organs  arise., 

The  inner  and  outer  germ-layers  are  primarily  simple  epithelial  structures, 
consisting  each  of  a  single  layer  of  cells.  This  primitive  characteristic  is  never 
wholly  obliterated  and  really  controls  all  of  the  modifications  which  these  two 
layers  undergo.  The  mesoderm,  on  the  other  hand,  is  primarily  not  epithelial, 
but  mesenchymal.  Mesenchyma  consists  of  widely  separated  cells  which  form  a 
continuous  network  of  protoplasm,  the  meshes  of  which  are  originally  filled  by  a 
homogeneous  intercellular  substance  or  matrix.  The  student  will  have  frequent 
occasion  in  his  practical  work  to  study  it  in  its  embryonic  stages. 


GERM-LA  YERS. 


35 


The  Ccelom. — The  coelom  is  the  primitive  body-cavity  of  the  embryo.  It 
arises  as  a  space  in  the  mesoderm.  As  soon  as  this  space  has  appeared  we  find 
that  the  cells  of  the  mesoderm,  which  bound  it,  assume  an  epithelial  character,  con- 
sequently the  mesoderm,  after  the  coelom  has  appeared,  consists  of  mesenchyma 
and  of  an  epithelial  layer  bounding  the  coelom.  This  epithelial  layer  is  called  the 
niesothelium.  The  mesoderm,  therefore,  differs  fundamentally  from  the  ecto- 
derm and  entoderm  by  this  peculiarity,  that  it  comprises  both  an  epithelial  and 
a  non-epithelial  portion.  Both  portions  play  very  important  roles  in  the  pro- 
duction of  the  various  tissues  and  organs  of  the  body.  There  is  another  respect 
also  in  which  the  mesoderm  differs  from  the  other  germ-layers,  for  we  find  that  it 
increases  in  volume  and  in  complexity  as  we  ascend  from  the  lower  to  the  higher 
types  of  animals,  or  as  we  pass  from  the  embryo  toward  the  adult  condition, 
more  than  does  either  the  outer  or  inner  germ-layer. 


CLASSIFICATION    OF   THE   TISSUES. 


(A)    ECTODKRMAL. 

Epidermis. 

a.  epidermal  appendages, 

b.  lens  of  eye. 
Epithelium  of 

a.  cornea, 

b.  olfactory  chamber, 

c.  auditory  organ, 
r/.  mouth 

(oral  glands), 
(enamel  organ), 
(hypophysis), 
e.   anus, 

f.  chorion, 

foetal  placenta, 

g.  amnion. 
Nervous  system. 

a.  brain, 

optic  nerve, 
retina, 

b.  spinal  cord, 


(B)  MESODKRMAL. 

1.  Mesothelium. 

a.  epithelium  of 

peritoneum, 
pericardium, 
pleura, 
urogenital  organs. 

b.  striated  muscles. 

2.  Mesenchyma. 

a.  connective  tissue, 

smooth  muscle, 
pseudo-endothelium, 
fat-cells, 
pigment  cells, 

b.  blood, 

c.  blood-vessels, 
</.   lymphatics, 

e.    spleen, 

f.  supporting  tissues, 

cartilage, 
bone, 

g.  marrow. 


d.    neuraxons. 


(C)  ENTODHRMAL. 

1.  Notochord. 

2.  Epithelium  of 

a.  digestive  tract, 

oesophagus, 

stomach, 

liver, 

pancreas, 

small  intestine, 

yolk-sack, 

large  intestine, 

caecum, 

vermix, 

rectum, 

allantois  (bladder), 

b.  pharynx, 

Eustachian  tube, 

tonsils, 

thymus, 

parathyroids, 

thyroid, 

c.  respiratory  tract, 

larynx, 

trachea, 

lungs. 


The  Specific  Quality  of  the  Germ-layers. — Each  germ-layer  has  its  specific 
and  exclusive  function  in  the  production  of  tissues,  giving  rise  only  to  the  tissues 
which  are  proper  to  it,  and  never  to  the  tissues  which  are  proper  to  either  of  the 
other  layers.  We  must,  indeed,  so  far  as  our  present  knowledge  goes,  regard 


36  GENERAL   CONCEPTIONS. 

the  cells  in  the  germ-layers  as  originally  wholly  indifferent  as  individual  cells. 
But  we  must,  nevertheless,  not  forget  that  as  members  of  a  germ-layer,  their 
potential  fate  is  already  restricted.  It  is  probable,  if  we  could  successfully 
transplant  an  undifferentiated  cell  from  one  germ-layer  to  another,  that  it  could 
take  part  in  the  production  of  the  tissues  proper  to  that  layer.  But  it  is  further 
probable  that  this  would  be  impossible  after  the  differentiation  of  the  cells  in 
any  layer  had  fairly  begun.  The  accompanying  table  presents  the  principal 
tissues  classified  according  to  the  layers  to  which  they  belong.  Or,  as  we  may 
say,  according  to  their  layership,  a  word  which  is  proposed  to  indicate  the  mem- 
bership of  a  given  cell  or  tissue  in  the  germ-layer  to  which  it  belongs.  The  layer- 
ship  of  a  cell  is  never  changed  after  the  differentiation  of  the  three  primitive 
layers  has  been  accomplished.  There  have  been  classifications  of  organs  on  the 
layership  basis  published  before,  but  inasmuch  as  organs  usually  contain  cells 
from  two  layers,  we  get  a  more  correct  presentation  of  the  actual  genetic  rela- 
tionship by  confining  our  tabulation  to  the  tissues.  Leucocytes  do  not  appear 
in  the  table  for  the  reason  that  their  first  origin  is  uncertain.  Blood-vessels 
arise  very  early,  before  the  clear  separation  of  the  mesoderm  and  entoderm  has 
occurred.  It  is  possible  that  they  are  entodermal.  With  these  two  limita- 
tions the  table  presents  our  present  knowledge. 

The  Constitution  of  Organs. — The  layership  of  most  organs  is  not  simple,  for 
as  we  find  organs  in  the  vertebrate  body  they  usually  consist  of  two  parts,  one 
of  which  may  be  regarded  as  the  part  proper  of  the  organ,  upon  which  the  per- 
formance of  its  special  function  directly  depends,  and  the  accessory  part,  which 
supplies  the  necessary  physiological  conditions  for  the  functioning  of  the  organ. 
For  example :  in  a  salivary  gland  the  actual  work  of  secretion  is  performed  by 
the  epithelial  cells  of  the  gland,  but  these  cells  cannot  act  unless  they  are  sup- 
ported by  connective  tissue  and  supplied  with  blood  and  lymph,  three  conditions 
which  depend  upon  the  mesoderm,  and  also  supplied  with  nerves,  a  condition 
which  depends  upon  the  ectoderm.  By  far  the  majority  of  organs  have  their 
functional  part  produced  from  epithelium,  and  this  epithelium  may  come  either 
from  the  original  outer  or  inner  germ-layer,  as  the  case  may  be,  or  from  the  meso- 
thelial  portion  of  the  middle  layer.  But  the  organ,  as  a  whole,  requires  for  its 
completion  the  addition  of  other  elements,  as  indicated  in  the  example  given. 
We  find,  therefore,  that  there  are  no  adult  organs  which  are  constituted  solely 
by  either  the  ectoderm  or  entoderm,  although  there  are  organs,  the  principal 
part  of  which  may  come  from  one  or  the  other  of  these  germ-layers,  but  to  com- 
plete the  organ  the  mesoderm  must  help.  On  the  other  hand,  the  mesoderm 
may  form  complete  organs  by  itself,  or  at  least  with  no  other  aid  from  the  other 
germ-layers  than  is  given  by  the  supplying  of  nerve-fibers.  Such  purely  meso- 
dermal  organs  are  illustrated  by  the  spleen,  the  kidney,  and  the  sexual  glands. 


THE  RELA  TIONS  OF  SURFA  CE  TO  MASS.  37 

The  Relations  of  Surface  to  Mass. 

However  much  the  weight  of  an  animal  increases  during  its  development, 
the  ratio  of  the  free  surface  to  the  mass  alters  but  slightly  from  the  ratio  estab- 
lished when  the  embryo  begins  to  take  food  from  outside.  It  is  only  for  conve- 
nience that  I  express  this  law  in  this  precise  form ;  in  reality,  about  it  our  knowl- 
edge is  scanty  and  our  conceptions  vague.  According  to  a  geometrical  principle, 
when  the  bulk  of  a  body  bounded  by  a  simple  surface  increases,  the  surface 
enlarges  less  than  the  mass — in  the  simplest  case  of  a  cube,  the  surface  increases 
as  the  square,  the  mass  as  the  cube,  of  the  diameter.  If  in  a  cube  of  unit  diam- 
eter one  unit  of  surface  bounds  one  unit  of  mass,  then  in  a  cube  of  three  units 
diameter  nine  units  of  surface  will  bound  twenty-seven  units  of  mass,  the  pro- 
portion in  the  first  cube  is  i :  i,  in  the  second  1:3.  To  maintain  the  proper 
proportion  in  the  embryo,  simple  enlargement  is  insufficient,  therefore  the  sur- 
face increases  by  becoming  more  and  more  irregular.  The  irregularities  are 
characteristic  of  each  organ  and  part,  and  may  be  either  large  or  microscopic. 
They  may  be  conveniently  grouped  under  two  main  heads — projections  and 
invaginations. 

Projections  are  illustrated  by  the  limbs,  filaments  of  the  gills  in  fishes,  the 
villi  of  the  intestine,  folds  of  the  stomach  in  ruminants,  etc.  In  every  case  the 
projection  is  covered  by  an  epithelium  and  has  a  core  of  mesodermic  tissue. 

In-vaginations  exist  in  much  more  varied  form  and  play  a  principal  part  in 
the  differentiation  of  the  animal  body.  They  may  be  classified  under  four 
principal  heads:  (i)  Dilatations;  (2)  diverticula;  (3)  glands;  (4)  vesicles.  Di- 
latations have  considerable  importance  in  embryology;  the  stomach,  lungs, 
bladder,  and  uterus  arise  as  gradual  dilatations  of  canals  or  tubes  of  originally 
nearly  uniform  diameters.  Di-verticula,  in  the  sense  of  relatively  large  blind 
pouches,  also  form  important  organs,  such  as  the  caecum  and  appendix  vermi- 
formis,  or  the  gall-bladder;  these  structures  arise,  each  as  a  blind  outgrowth  of  a 
canal,  the  walls  of  which  at  a  certain  point  rapidly  grow  to  form  the  pouch. 
Glands  are,  as  first  shown  by  Johannes  Miiller's  classic  researches,  only  small 
diverticula,  which  end  blindly  and  appear  in  an  immense  variety  of  modifica- 
tions ;  the  manifold  types  of  glands  are  discussed  below  in  a  separate  paragraph ; 
they  constitute  the  largest  class  of  organs  with  which  we  have  to  deal.  The 
glands  are  developed  from  epithelium  and  push  their  way  into  the  mesoderm 
upon  which  the  epithelium  rests,  while  in  dilatations,  and  in  diverticula,  the 
epithelium  and  mesoderm  expand  together.  Vesicles  we  call  those  epithelial 
sacs  which  develop  somewhat  like  glands  by  growing  into  the  mesoderm,  but  the 
mouth  of  the  invagination  closes  by  the  coalescence  of  the  epithelium,  thus 
shutting  the  cavity.  The  closed  sac  separates  from  the  epithelium  from  which 


38  GENERAL   CONCEPTIONS. 

a 

it  arose,  and  connective  tissue  grows  between  the  two ;  the  sac  may  then  undergo 
various  modifications.  The  membranous  labyrinth  of  the  ear  is  developed  from 
the  ectoderm  in  this  way,  as  is  also  the  lens  of  the  eye.  We  might  perhaps  also 
class  the  medullary  canal  under  this  head  (cf .  Chap.  VHX)  if  we  choose  to  con- 
sider it  as  a  vesicle  so  much  lengthened  that  it  has  become  a  tube.  -^  ^ 

V 

The  Law  of  Unequal  Growth. 

The  changing  shapes  of  the  embryo  and  the  development  of  the  irregulari- 
ties— projections  and  invaginations — which  preserve  the  proper  proportion 
between  the  surface  and  the  mass  of  the  body,  both  depend  upon  the  unequal 
growth  of  the  germ-layers,  especially  in  superficies.  The  expansion  of  a  germ- 
layer  having  the  epithelial  type  of  structure  *  may  take  place  by  three  means : 
(i)  The  multiplication  of  the  cells;  (2)  the  flattening  out  of  the  cells;  (3)  en- 
largement of  the  cells.  In  the  early  stages  of  development  the  influence  of  the 
first  two  factors  predominates;  during  the  later  stages,  especially  after  birth, 
the  latter.  Of  the  three  factors,  the  first  is  the  most  important. 

The  unequal  multiplication  of  the  cells  in  all  embryonic  epithelia  is  the 
fundamental  factor  of  development,  and  we  see  it  shaping  the  embryo,  its  organs, 
and  the  parts  of  organs,  before  histological  differentiation  really  begins.  The 
distinct  areas  and  centers  of  growth  which  are  necessary  to  develop  the  human 
body  out  of  the  germ-layers  are  innumerable,  and  their  distribution,  limitations, 
and  interactions  make  up  a  large  part  of  the  subject-matter  of  embryology.  At 
every  turn  of  our  studies  we  encounter  fresh  illustrations.  If  in  a  limited  area 
of  a  cellular  membrane  there  occurs  a  growth  of  expansion  more  rapid  than  in 
the  neighboring  parts,  then  that  area  is,  as  it  were,  bounded  by  a  fixed  ring,  and 
can,  therefore,  find  room  for  its  own  expansion  only  by  rising  above  the  level  of 
the  membrane;  thus,  when  in  the  embryonic  region  of  the  blastodermic  vesicle 
the  growth  becomes  more  rapid,  the  embryo  begins  to  rise  above  the  level  of  the 
vesicle;  thus  when,  at  a  certain  point  of  the  surface  of  the  embryo,  a  steady  and 
long-continued  growth  occurs,  the  limb  appears,  gradually  lengthening  out,  and 
enlarges  from  a  small  bud  at  first  to  a  complete  arm  or  leg.  If  the  departure 
takes  place  the  other  way,  we  have  an  invagination  produced;  thus,  for  every 
hair  of  the  skin  and  for  every  gland  of  the  intestine  there  is  a  separate  center  of 
growth. 

The  reason  for  the  unequal  growth  is  unknown.  We  have  not  even  an  hypo- 
thesis to  offer  as  to  why  one  group  of  cells  multiplies  or  expands  faster  than 
another  group  of  apparently  similar  cells  close  by  in  the  same  germ-layer.  It  is 


*  By  this  limitation  we  exclude  the  mesenchyma,  but  not  the  mesothelium. 


GERM-CELLS.  39 

no  real  explanation  to  say  that  it  is  the  result  of  heredity,  for  that  leaves  us  as 
completely  in  the  dark  as  ever  as  to  the  physiological  factors  at  work  in  the  devel- 
oping individual. 

The  conception  that  the  development  of  an  animal  depends  fundamentally 
upon  the  unequal  expansion  and  consequent  foldings  and  bendings  of  the  germ- 
layers  was  first  suggested  by  the  researches  of  C.  F.  Wolff  on  the  development  of 
the  intestine,  and  was  more  clearly  recognized  by  Pander,  who  definitely  asserted 
that  the  formation  of  the  embryo  is  effected  by  foldings  of  the  germ-layers,  and 
the  truth  of  Pander's  view  was  conclusively  demonstrated  by  C.  E.  von  Baer  in 
1828.  In  recent  times  His  has  studied  the  problem  very  intently,  and  in  his 
memoir  on  the  chick  discussed  it  minutely.  In  this  memoir  is  to  be  found  most 
of  what  little  we  know  of  this  aspect  of  embryological  mechanics. 

Qerm=cells. 

Recent  investigations  have  made  it  probable  that  a  few  cells  are  set  apart 
during  the  period  of  segmentation  to  form  the  germ-cells.  Their  number  is 
small;  they  preserve  for  some  time  the  appearance  of  segmentation  spheres,  as 
the  cells  which  are  formed  during  the  segmentation  of  the  ovum  are  sometimes 
called.  They  multiply  very  slowly  during  the  earliest  stages  of  development. 
A  great  majority  of  the  cells  produced  during  segmentation  lose  the  character  of 
segmentation  spheres,  and  divide  rapidly  and  repeatedly.  The  cells  belonging 
to  the  class  of  this  majority  form  the  various  tissues  of  the  body.  The  germ- 
cells,  on  the  contrary,  seem  to  multiply  very  slowly  and  never  to  become  very 
numerous  in  the  embryo.  As  they  multiply  they  separate  from  one  another  and 
become  more  or  less  completely  surrounded  by  tissue  cells.  They  pursue  their 
development,  one  is  tempted  to  say,  independently  of  tissue  formation  and  some- 
what like  foreign  members.of  the  body.  We  put,  accordingly,  the  germ-cells  in 
a  class  by  themselves  in  contrast  to  the  body  or  somatic  cells. 

Our  actual  knowledge  of  the  history  of  the  germ-cells  is  very  incomplete. 
The  statements  just  made  about  them  are  based  on  observations  on  very  few 
animals.  Their  exact  origin  has  been  traced  only  in  three  species  of  vertebrates,  — , 
all  fishes,  the  teleosts  Cymatogaster  and  Micrometrus,  and  the  elasmobranch 
Squalus  acanthias.  In  these  three  forms  the  germ-cells  arise  during  segmenta- 
tion, remain  more  or  less  closely  together,  or  segregated,  during  the  earliest 
stages.  They  then  separate  from  one  another  and  gradually  migrate  into  the 
epithelium,  which  covers  the  anlage  of  the  genital  gland,  which  thus  becomes 
the  so-called  "germinal  epithelium." 

The  existence  of  the  germinal  epithelium  has  long  been  known,  and  its 
characteristics  have  been  described  in  all  recent  text-books  of  embryology.  The 


40  GENERAL   CONCEPTIONS. 

germ-cells  in  the  germinal  epithelium  are  commonly  known  by  the  name  of  the 
primitive  ova.  The  transformation  of  these  cells  into  true  ova  has  been  traced 
in  a  great  many  forms,  so  that  the  transformation  may  be  considered  as  de- 
monstrated conclusively  for  all  vertebrate  animals.  It  is  further  commonly 
assumed  that  the  germ-cells  or  primitive  ova  also  give  rise  to  the  male  elements, 
playing  in  the  formation  of  the  testes  a  role  similar  to  that  which  they  play  in 
the  ovary.  There  is,  unfortunately,  up  to  the  present  time,  no  conclusive  proof 
by  direct  observation  that  the  primitive  ova  are  the  actual  parents  of  the  cells 
which  give  rise  to  the  spermatozoa. 

When  a  germ-cell  is  transformed  into  an  ovum,  it  undergoes  great  enlarge- 
ment, its  nucleus  is  modified,  the  protoplasm  is  changed  in  appearance  and  be- 
comes loaded  with  yolk  granules,  and  over  the  surface  of  the  cell  appear  two 
membranes,  an  inner  very  thin  one,  called  the  mtelline  membrane,  and  an  outer 
much  thicker  one,  known  as  the  zona  pellucida.  (For  a  fuller  description  see 
page  33:)  We  thus  learn  that  the  germ-cells  preserve  their  resemblance  to 
segmentation  spheres  only  during  embryonic  life.  When  they  become  ova, 
they  pass  through  a  series  of  important  changes  in  their  organization.  If  it  is 
true  that  these  germ-cells  also  give  rise  to  the  male  elements,  then  we  must 
further  say  that  in  order  to  produce  those  elements  the  germ-cells  pass  through 
another  series  of  profound  changes. 

It  is  further  known  that  in  order  to  evolve  the  sexual  elements,  both  male 
and  female,  the  cell  which  is  to  produce  them  divides  twice,  and  in  a  special 
manner,  which  we  designate  by  the  term  "reduction  division."  This  process  is 
described  in  all  the  recent  text-books  of  cytology  and  histology.  It  does  not 
fall  within  the  scope  of  this  work,  which  deals  with  embryology  in  the  strict  sense 
only. 

The  Theory  of  Heredity. 

We  owe  to  Moritz  Nussbaum  the  theory  of  germinal  continuity — the  only 
theory  of  heredity  which  seems  tenable  at  the  present  time.  According  to  this 
theory,  the  germ-cells  are  set  aside  during  the  segmentation  of  the  ovum  and 
preserve  the  essentially  undifferentiated  qualities  of  the  protoplasm  and  nucleus 
of  the  ovum,  from  the  division  of  which  they  arise.  Just  as  the  cells  formed 
during  segmentation  are  capable  of  producing  the  various  tissues  of  the  body,  so 
the  germ-cells  have  and  preserve  this  faculty.  If  we  term  the  material  of  the 
original  ovum  germ-plasm,  we  may  say  that  this  germ-plasm  gives  rise  to  the 
various  tissue-forming  cells  which  make  up  the  body.  And  by  this  very  con- 
version into  tissue  cells,  that  germ-plasm  is  changed,  and  is  no  longer,  as  we  have 
learned  before,  capable  of  the  full  range  of  development.  The  germ-cells,  on  the 


THE  LAW  OF  RECAPITULATION.  41 

contrary,  do  remain  so  capable,  and  it  is  precisely  in  order  to  preserve  this  capac- 
ity that  they  hold  aloof  from  the  formation  of  the  body  tissues  and  pursue  their 
own  independent  career.  A  portion  of  the  germ-plasm  of  the  parent  ovum  is, 
so  to  speak,  short-circuited  into  the  genital  elements  which  produce  the  offspring. 

If  we  accept  this  view,  we  are  forced  to  make  the  supplementary  hypothesis 
that  the  conspicuous  complicated  changes,  by  which  the  germ-cells  are  converted 
into  sexual  elements,  do  not  involve  the  differentiation  in  the  true  sense — i.  e., 
strictly  comparable  to  that  which  we  observe  in  the  somatic  cells.  Although 
this  hypothesis  seems  a  logical  necessity  of  the  theory  of  germinal  continuity, 
we  cannot  at  present  verify  it  by  any  observed  facts.  The  only  other  theory  of 
heredity  which  has  ever  been  seriously  considered  is  that  of  pangenesis,  which 
was  formulated  by  Darwin,  whose  words  I  quote : 

"  But  besides  this  means  of  increase  I  assume  that  cells,  before  their  conver- 
sion into  completely  passive  or  'form-material,'  throw  off  minute  granules  or 
atoms,  which  circulate  freely  throughout  the  system,  and  when  supplied  with 
proper  nutriment  multiply  by  self-division,  subsequently  becoming  developed 
into  cells,  like  those  from  which  they  were  derived.  These  granules,  for  the  sake 
of  distinctness,  may  be  called  ce'1-gemmules,  or,  as  the  cellular  theory  is  not  fully 
established,  simply  gemmules.  They  are  supposed  to  be  transmitted  from  the 
parents  to  the  offspring,  and  are  generally  developed  in  the  generation  which 
immediately  succeeds,  but  are  often  transmitted  in  a  dormant  state  during 
many  generations,  and  are  then  developed." 

Many  modifications  of  this  theory  have  been  proposed  by  speculative 
writers,  and  many  different  names  have  been  bestowed  upon  the  gemmules  of 
Darwin  according  to  the  fancy  of  each  author  and  the  particular  set  of  qualities 
which  he  attributed  to  these  imaginary  particles.  Such  views  attained  their 
culmination  in  Weismann's  complicated  useless  hypotheses.  All  of  these  specu- 
lations have  only  an  historical  interest,  having  proved  themselves,  from  a  scien- 
tific standpoint,  to  be  absolutely  barren. 

The  Law  of  Recapitulation. 

This  law  as  commonly  formulated,  is  that  the  development  of  the  indi- 
vidual recapitulates  the  development  of  the  race,  or,  in  other  words,  the  ontogeny 
recapitulates  the  phytogeny.  This  way  of  stating  the  law  is  in  so  far  objectionable 
that  it  presents  the  theoretical  interpretation  of  the  law  rather  than  the  actual 
generalization  of  the  facts.  The  essential  datum  upon  which  the  law  is  based 
is  that  the  embryo  of  a  given  animal  has  striking  morphological  resemblances  to 
the  adult  forms  of  lower  allied  types.  Since  the  theory  of  evolution  was  estab- 
lished by  Darwin  this  resemblance  has  been  interpreted  as  due  to  the  inheritance 


42  GENERAL   CONCEPTIONS. 

of  ancestral  characters  appearing  in  the  embryo.  The  embryo  is  looked  upon  as 
the  representative  of  the  actual  ancestor  by  modification  of  which  the  adult  form 
was  evolved.  It  is  further  assumed  that  the  change  of  the  embryo  into  the 
adult  type  follows  the  same  general  course  as  the  development  of  the  remote 
ancestor  into  the  particular  species  under  consideration.  Speaking  broadly, 
this  interpretation  is  undoubtedly  justifiable.  If  it  were  exactly  true,  it  would 
be  necessary  only  to  know  the  embryology  of  an  animal  in  order  to  establish 
the  evolution  of  its  species.  Experience,  however,  very  quickly  demonstrates 
that  this  procedure  is  by  no  means  possible,  because  the  embryo  is  not  a  correct 
or  adequate  record  of  the  ancestral  type.  It  is  inadequate  chiefly  for  three 
reasons :  First,  because  the  embryo  has  necessities  of  its  own,  and  in  the  course 
of  evolution  embryos  acquire  special  peculiarities  by  which  they  become  adapted 
to  the  conditions  of  their  life.  Such  changes  in  organization  do  not  correspond 
to,  but  on  the  contrary  diverge  from,  the  inherited  ancestral  traits,  and  in  so  far 
as  they  are  present  they  mask  or  alter  those  structural  features  of  the  embryo 
which  represent  the  ancestral  record.  Second,  because  the  embryos  consist  of 
undifferentiated  cells  (q.v.).  Now,  the  adult  ancestors  representing  lower  types 
of  organization  of  course  had  differentiated  tissues,  which  enabled  them  to 
perform  the  functions  of  adult  life.  One  of  the  first  things  which  will  impress 
itself  upon  the  student  of  vertebrate  embryology  is  that  though  he  may  find 
at  the  proper  stage  in  the  embryo  the  organs  of  the  body  clearly  developed, 
yet,  owing  to  the  fact  that  they  consist  of  relatively  undifferentiated  cells,  they 
are  incapable,  in  large  part,  of  performing  the  functions  which  they  are  ulti- 
mately to  assume,  and  the  performance  of  which  is  the  very  object  of  their  de- 
velopment. This  change  in  histological  structure  brings  about  a  marked  unlike- 
ness  of  the  embryo  to  the  assumed  ancestral  type.  Third,  the  embryo  at  each 
stage  of  its  development  must  be  regarded  as  the  mechanical  cause  of  the  next 
and  of  all  following  stages.  It  must  necessarily,  therefore,  have  in  itself  pecu- 
liarities by  which  it  is  distinguished  from  all  other  embryos.  It  is  impossible, 
accordingly,  that  all  embryos  should  be  alike.  It  is  only  necessary  for  the  stu- 
dent to  compare  embryos  of  various  vertebrates  one  with  another  to  satisfy 
himself  that  they  have  conspicuous  distinctive  characteristics.  When  our 
knowledge  shall  have  grown  sufficiently,  we  shall  be  able  to  classify  vertebrates 
by  their  embryos  as  perfectly,  or  perhaps  even  more  perfectly,  than  we  can  by 
the  consideration  of  the  adult  forms.  Every  embryo  is  modified  from  the  very 
start  away  from  the  assumed  ancestral  organization,  in  order  that  its  peculiarities 
may  cause  it  mechanically  to  produce  the  new  form  which  has  been  evolved. 

In  some  of  the  invertebrate  animals — as,  for  instance,  among  the  hydroids 
and  jellyfishes — the  law  of  recapitulation  can  be  much  more  easily  verified  than 


THE  LAW  OF  RECAPITULATION.  43 

in  the  higher  forms  which  have  purely  embryonic  types  of  development.  From 
what  has  been  said,  it  will  be  recognized  that  the  likeness  of  the  embryo  to  the. 
adult  lower  form  is  a  general  morphological  resemblance  only,  not  an  exact  one, 
and  that  therefore  it  is  extremely  difficult  to  infer  from  the  embryonic  organiza- 
tion what  the  ancestral  type  was.  Hitherto  all  phylogenetic  inferences  drawn 
by  embryologists  have  been  largely  speculative  in  character,  and,  it  may  be 
added,  have  been  more  remarkable  for  their  number  and  variety  than  for  their 
value. 

The  resemblance  between  embryos  and  lower  adult  forms  has  been  known 
for  a  century  past.  It  was  first  adequately  asserted  in  181 1  by  J.  F.  Meckel,  and 
since  then  has  been  constantly  discussed.  More,  perhaps,  was  done  to  empha- 
size it  by  Louis  Agassiz  than  by  any  one  else.  Von  Baer,  the  creator  of  modern 
scientific  embryology,  called  attention  in  1828  to  the  limitations  which  must 
necessarily  be  put  upon  Meckel's  generalization.  It  is  to  be  regretted  that  von 
Baer's  wise  thought  on  this  subject  has  not  been  more  appreciated.  He  put 
forth  four  generalizations :  First,  that  which  is  common  to  a  large  group  of  ani- 
mals develops  in  the  embryo  earlier  than  that  which  is  special;  second,  from 
the  most  generalized  stage  structures  less  generalized  are  developed,  and  so  on 
until  finally  the  most  special  appears;  third,  the  embryo  of  a  given  animal  form, 
instead  of  passing  through  the  other  given  forms,  separates  itself  from  them 
more  and  more;  fourth,  therefore,  essentially  the  embryo  of  the  higher  forms  is 
never  like  a  lower  form,  but  only  like  its  embryo.  The  first  to  point  out  the 
possible  phylogenetic  significance  of  these  facts  with  perfect  clearness  was  Fritz 
Miiller,  in  a  little  book  entitled  "Fur  Darwin,"  published  in  1864.  Ernst 
Haeckel  took  up  this  interpretation  and  secured  wider  attention  for  it.  He 
termed  the  law  of  recapitulation  the  "  biogenetic  law."* 

The  student  will  encounter  in  his  practical  study  many  illustrations  of  the 
resemblances  which  we  have  been  discussing,  so  that  it  is  unnecessary  here  to  do 
more  than  mention  a  few  for  the  purpose  of  illustration.  In  the  embryos  of 
birds  and  mammals  the  pharynx  forms  a  series  of  lateral  pouches  which  we  know 
as  the  gill  pouches,  and  which  develop  in  the  same  way  as,  resemble  strikingly, 
and  are  homologous  with,  the  gill  pouches  of  fishes,  which  in  the  fishes  give  rise 
to  the  so-called  gill  clefts.  The  heart  of  a  young  mammalian  embryo  is  a  simple 
tube  with  only  a  single  continuous  cavity  resembling  the  heart  of  the  lower 
fishes.  The  embryonic  kidney  or  Wolffian  body  of  man  resembles,  and  is 
homologous  with,  the  kidney  of  the  frog,  but  it  disappears  almost  completely 
before  adult  life.  These  few  examples  may  suffice. 


*  "  Biogenetisches  Grundgesetz." 


mi 


CHAPTER   II. 
THE  EARLY  DEVELOPMENT  OF  MAMMALS. 

The  Spermatozoon. 

The  spermatozoa  of  mammals  are  filaments  consisting  of  a  short  thick  end 
called  the  head,  and  a  very  long,  delicate  thread  called  the 
tail.  They  are  of  minute  size  as  compared  with  the  ovum. 
The  head  varies  greatly  in  shape  according  to  the  species. 
It  contains  chromatin,  hence  it  stains  darkly  with  those 
histological  dyes  which  color  nuclei.  The  tail  consists  of 
three  parts:  First,  the  middle  piece,  which  is  next  the  head, 
is  short,  and  the  thickest  of  the  three  parts,  contains  an  axial 
thread,  and  probably  always  has  a  very  fine  -spiral  thread  run- 
ning round  it;  second,  the  main  piece,  which  is  the  longest 
part  of  the  tail;  and,  third,  the  end  piece,  which  is  not  more 
than  a  line,  even  as  seen  with  very  high  microscopic  powers. 
The  Human  Spermatozoon. — The  human  spermatozoon  is 
0.055  mm-  l°ng — the  head  being  0.005  mm.,  the  tail  0.050, 
and  the  middle  piece  0.009.  It  is  shown  in  two  views  in 
Fig.  i.  The  head  is  flattened  and  pointed.  Seen  from  the 
flat  side,  it  appears  oval  (Fig.  i,  A]  with  the  front  end  gener- 
ally tapering  a  little,  but  never  pointed.  The  anterior  half 
or  two-thirds  has  a  brighter  and  more  transparent  part. 
Seen  on  edge  (Fig.  i,  B),  the  head  has  a  pointed  form  with  a 
posterior,  thicker,  round  dark  part.  By  adjustment  of  the 
focus  it  can  be  ascertained  that  the  sides  near  the  point  are 
depressed,  somewhat  like  those  of  human  red  blood-corpuscles. 
Some  writers  maintain  that  there  is  a  special  tip  projecting 
from  the  head  as  a  cylinder  thread,  with  a  hook  at  its  end. 
The  middle  piece,  mi,  is  directly  united  with  the  head  by  a 
transverse  joint.  It  is  cylindrical  and  about  as  long  as,  or  a 
little  longer  than,  the  head.  Its  surface  is  often  granular  or 
rough,  and  there  cling  to  it  a  few  shreds  of  protoplasm.  It  has  a  spiral 

44 


FIG.  i.  —  HUMAN 
SPERMATOZOA. 

A,  Complete  sperma- 
tozoon. B,  Head 
seen  from  the  side. 
C,  Extremity  of  the 
tail.  A,  H<ad.  mi, 
Middle  piece,  m, 
Main  piece,  e,  End 
piece.  All  highly 
magnified. — {After 
Retzius.  \ 


THE  FULL  Y  GRO  WN  O  VUM  BEFORE  MA  TURA  TION.  4  5 

thread,  which  is  easily  overlooked  on  account  of  its  extreme  fineness.  The 
main  piece,  m,  of  the  tail  is  about  half  as  thick  as  the  middle  piece.  It 
gradually  tapers  and  ends  abruptly  at  the  beginning  of  the  still  finer  and  very 
short  end  piece,  e. 

The  spermatozoa,  when  free  in  the  fluids  in  which  they  normally  occur,  are 
capable  of  active  locomotion.  This  is  achieved  by  means  of  the  tail,  which  acts 
as  the  swimming  organ  by  vibratory  undulations  which  drive  the  spermatozoon 
along,  head  foremost.  The  tail  has  often  been  compared  to  the  flagellum  which 
serves  as  the  locomotive  organ  for  many  of  the  unicellular  organisms. 

The  Fully  Grown  Ovum  Before  Maturation. 

The  structure  to  be  here  described  is  not  the  true  sexual  element,  but  is  only 
the  modified  germ-cell  which  has  accomplished  its  period  of  growth  and  is  ready 
to  be  transformed  into  the  genuine  female  sexual  element.  This  transformation 
is  called  the  maturation,  and  is  accomplished  essentially  by  the  expulsion  of  the 
so-called  polar  granules.  The  full-grown  mammalian  ovum  is  found  in  the 
ovary  in  the  center  of  the  discus  proligerus  of  the  Graafian  follicle.  It  measures 
from  o.io  to  0.15  mm.  in  diameter.  It  is  approximately  spherical.  In  some 
cases  observers  have  found  a  very  delicate  vitelline  membrane  covering  the  pro- 
toplasm. Others  have  failed  to  observe  this.  Outside  there  is  a  thick  envelope 
measuring  from  0.02  to  0.03  mm.  in  diameter  and  known  as  the  zona  pellucida  or 
radiata.  Against  the  outside  of  the  zona  rest  the  cells  of  the  discus  proligerus 
which  constitute  the  so-called  "  corona  radiata."  The  nucleus  is  large,  spherical, 
contains  a  distinct  nucleolus,  and  always  occupies  an  eccentric  position.*  The 
protoplasm  of  the  cell  is  large  in  amount,  granular  in  appearance,  forms  a  dis- 
tinct reticulum,  and  contains  a  greater  or  less  number  of  yolk  granules  which 
vary  considerably  in  character,  size,  and  distribution  in  different  mammals. 
They  are  usually  more  or  less  concentrated  in  the  central  portion  of  the  ovum, 
leaving  the  outer  portion,  known  as  the  protoplasmic  zone,  more  or  less  free. 

The  Human  Ovum. — The  full-grown  human  ovum  is  distinguished  among 
mammalian  ova  for  the  clear  development  and  ready  visibility  of  all  its  parts,  a 
peculiarity  due  chiefly  to  the  small  amount  of  the  yolk  and  the  fewness  of  the 
fat  granules  it  contains.  Fig.  2  represents  an  ovum  from  a  woman  of  thirty 
years.  The  specimen  was  obtained  by  ovariotomy,  examined  and  drawn  in  the 
fresh  state,  being  in  the  liquor  folliculi.  The  specimen  gave  the  following  meas- 
ures: The  diameter  of  the  whole  ovum,  including  the  zona  radiata,  165-170  /-«; 
thickness  of  zona,  20-34  !l  '>  perivitelline  fissure,  i  .3  n ;  the  clear  outer  zone  of  the 


*  The  nucleus  was  formerly  termed  "  germinal  vesicle  "  ;  the  nucleolus,  "  germinal  spot." 


46 


THE  EARL  Y  DE  VEL  OPMENT  OF  MAMMALS. 


PI. 


yolk,  4-6  n;  the  protoplasmic  zone,  10-21  //;  the  zone  of  yolk  granules,  82-87  /;; 
nucleus,  25-27  /;.  The  corona  radiata,  cor.  r,  consists  of  elongated  radiating  cells 
with  rounded  ends  and  oval  nuclei.  The  zona  pellucida  shows  a  distinct  radial 
striation.  This  is  probably  due  to  the  presence  of  minute  canals  running  through 
the  zona.  The  ovum  proper  is  separated  by  a  narrow  fissure,  the  perivitelline 
space,  from  the  zona,  within  which  it  lies  free  and  loose.  Hence  when  a  fresh 
specimen  is  examined,  the  same  side  of  the  ovum,  that  containing  the  nucleus 
and  which  is  the  lightest  part,  is  always  found  uppermost.  In  this  ovum  no 

vitelline  membrane  was  observed.  The 
body  of  the  ovum  may  be  divided  into 
an  inner  kernel  containing  the  yolk 
granules,  and  an  outer  protoplasmal 
zone,  of  which  the  very  thin  outermost 
layer  is  clear  and  therefore  more  or  less 
differentiated  from  the  broader,  deeper 
layer,  which  is  granular  and  constitutes 
most  of  the  zone,  PI.  The  yolk  grains 
are  i  [J.  or  less  in  diameter.  They  are 
highly  refringent  and  of  various  kinds. 
Their  characteristics  have  not  yet  been 
accurately  investigated.  The  nucleus 
is  nearly  spherical  and  has  a  conspicu- 
ous nucleolus.  In  fresh  specimens  the 
nucleolus  shows  amoeboid  movements, 
even  at  ordinary  summer  temperatures, 
for  several  hours  after  removal  from 
the  ovary.  It  is  only  in  hardened  speci- 
mens that  the  reticulum  of  the  nucleus  can  be  clearly  observed. 


FIG.     2. — FULL-GROWN     HUMAN    OVUM     BEFORE 

MATURATION. 
cor.r,   Part  of  corona  radiata.         Z,    Zona  pellucida. 

PI,    Protoplasm.        Y,   Yolk.       me,    Nucleus. — 

(After  IV.  Nagel. ) 


Ovulation. 

The  discharge  of  the  ovum  from  the  ovary  is  called  ovulation.  It  results 
from  structural  changes  in  the  Graafian  follicle,  and  these  changes  continue  after 
the  departure  of  the  ovum,  transforming  the  Graafian  follicle  into  a  so-called 
corpus  luteum.  The  exact  history  of  these  changes  does  not  fall  within  the  scope 
of  this  work.  The  essential  steps  in  the  process  are  the  growth  of  the  Graafian 
follicle  and  the  thinning  of  its  wall  at  a  point  at  the  surface  of  the  ovary.  The 
thin  part  is  called  the  stigma.  This  breaks  through  and  establishes  an  opening 
by  which  the  ovum  surrounded  by  the  corona  radiata,  together  with  the  liquor 
of  the  follicle,  can  escape  into  the  periovarial  chamber,  whence  it  makes  its  way 
into  the  Fallopian  tube.  The  growth  of  tissue  in  the  walls  of  the  collapsed 


THE  MATURATION  OF  THE   OVUM.  47 

Graafian  follicle  fills  up  the  space  of  the  same,  constituting  a  mass  which  is 
known  as  the  corpus  luteum  on  account  of  its  yellow  color.  The  most  character- 
istic elements  of  this  structure  are  the  large  cells  which  contain  the  pigment. 
Each  cell  has  a  rounded  nucleus  and  a  large  protoplasmic  body,  which  is  also 
more  or  less  rounded  in  form.  The  lutein  granules  are  in  these  cells.  The 
function  of  the  corpus  luteum  was  long  entirely  unknown.  Recently  the  theory 
has  been  suggested  by  Born  that  these  cells  exert  an  influence  upon  the  uterus 
by  which  it  is  prepared  to  receive  the  ovum.  This  influence  may  be  suggested  to 
act  by  means  of  a  chemical  substance  produced  by  the  lutein  cells  and  added  to 
the  blood,  which  then  affects  the  uterus.  There  are  some  experimental  observa- 
tions tending  to  prove  the  correctness  of  this  theory. 

The  brilliant  color  of  the  corpus  luteum  is  especially  characteristic  of  man, 
and  has  determined  the  name  of  the  structure.  In  sheep  the  pigment  is  pale 
brown,  in  the  cow  dark  orange,  in  the  mouse  brick  red,  in  the  rabbit  and  pig 
flesh-color. 

The  Maturation  of  the  Ovum. 

Maturation  is  the  term  applied  to  the  series  of  changes  by  which  the  fully 
grown  egg-cell  is  transformed  into  a  true  female  sexual  element.  Viewed  exter- 
nally in  the  living  ovum,  the  process  manifests  itself  chiefly  by  the  separating  off 
of  one,  or  usually  two,  small  bodies  of  protoplasm,  each  of  which  contains  some 
nuclear  material.  These  small  bodies  are  generally  known  by  the  name  of  polar 
globules.  They  take  no  further  part  in  the  development,  ultimately  disintegrate, 
and  are  lost.  The  remaining  ovum  is  capable  of  impregnation.  It  is  now 
known  that  the  production  of  the  polar  globules  is  the  result  of  a  special  form  of 
cell  division,  which  we  term  the  "reduction  division."  When  the  first  polar 
globule  is  formed,  the  egg-cell  divides  into  one  very  large  cell  and  a  second  very 
small  one.  When  the  second  polar  globule  is  formed,  the  larger  of  the  cells  again 
divides,  producing  a  second  small  cell  and  a  new  large  one.  This  large  one  is  the 
true  female  element.  When  an  ovum  is  about  to  mature,  its  nucleus  moves 
nearer  that  point  of  the  surface  which  may  be  regarded  as  the  center  of  the  so- 
called  animal  pole,  or  region  of  the  ovum,  which  contains  most  of  the  protoplasm 
and  less  of  the  yolk  material.  During  the  migration  of  the  nucleus,  the  cell  as  a 
whole  usually  contracts  so  that  a  space  appears  between  it  and  the  zona  radiata. 
Concerning  the  force  that  moves  the  nucleus  we  have  no  definite  knowledge. 
When  near  the  surface,  the  nucleus  as  such  disappears.  Older  writers  supposed 
that  it  was  lost  altogether,  but  we  now  know  that  the  disappearance  of  the 
nucleus  is  only  apparent,  not  actual,  being  in  reality  a  metamorphosis.  It  is 
probable  that  the  first  step  is  the  discharge  of  the  nuclear  fluid  into  the  surround- 


48  THE  EARLY  DEVELOPMENT  OF  MAMMALS. 

ing  protoplasm,  causing  the  nucleus  to  become  more  or  less  shriveled.  The 
second  step  is  the  dissolution  of  the  membrane  of  the  nucleus  so  that  the  nuclear 
contents  are  brought  into  direct  contact  with  and  partly  mixed  with  the  proto- 
plasm of  the  cells.  The  third  step,  which  in  time  more  or  less  accompanies  the 
second,  is  the  gathering  of  the  chromatin  of  the  nucleus  into  a  definite  number  of 
separate  granules  or  chromosomes.  These  chromosomes  are  always  conspicuous 
and  are  larger  than  those  formed  during  ordinary  cell  division.  Their  number  is 
also  highly  characteristic.  As  is  now  well  known,  there  appear  during  the  process 
of  indirect  cell  division  a  fairly  definite  number  of  chromosomes,  a  number  which 
is  characteristic  for  each  species.  In  numerous  cases  it  has  been  observed  that 
the  number  of  chromosomes  in  the  maturing  egg-cell  is  exactly  one-half  of  that 
found  during  the  ordinary  cell  divisions  of  the  same  species.  The  chromatin 
granules  lie  at  first  irregularly.  Fourth,  there  arises  a  characteristic  spindle 
figure  such  as  accompanies  mitosis.  The  chromatin  forms  an  equatorial  plate, 
each  granule  being  associated  with  one  of  the  spindle  threads.  The  shape  of  the 
spindle  varies,  as  does  also  the  distribution  of  the  granules  of  the  equatorial  plate. 
In  guinea-pigs  the  ends  of  the  spindle  are  pointed  and  the  threads  are  straight, 
the  outline  of  the  spindle  being  like  a  diamond ;  in  the  bat  the  spindles  are  barrel- 
shaped  and  the  threads  are  curved.  In  many  cases  it  is  possible,  and  it  may  be 
found  to  be  true  of  all  cases,  that  the  axis  of  the  spindle  is  at  right  angles  to  the 
radius  of  the  ovum.  The  nuclear  spindle  now  changes  its  position,  becomes  first 
oblique,  and  then  radial.  One  end  of  the  spindle  lies  close  to  the  surface  of  the 
ovum.  The  first  step  is  the  division  proper.  The  spindle,  driven  by  an  undis- 
covered power,  moves  centrifugally  until  it  is  partly  extruded  from  the  egg. 
The  projecting  end  is  enclosed  in  a  distinct  mass  of  protoplasm  which  gradually 
increases  and  soon  becomes  constricted  around  its  base.  The  fragments  of  chro- 
matin have  each  divided  into  two  parts,  and  one-half  of  each  fragment  moves 
toward  one  end,  and  the  other  half  toward  the  other  end  of  the  spindle.  The 
half  fragments  of  each  set  move  together,  hence  there  seem  tq  be  two  plates 
within  the  spindle.  The  translation  of  the  groups  of  chromatin  continues  until 
they  reach  the  end  of  the  spindle.  The  achromatic  threads  then  break  through 
in  the  middle,  and  thus  the  original  nucleus,  or  at  least  a  part  of  it,  has  been 
divided.  There  are  now  two  masses  of  nuclear  substance,  one  in  the  ovum,  the 
other  in  the  polar  globule.  The  result  of  the  whole  process  is  the  formation  of 
two  cells  extremely  unequal  in  size,  and  each  containing  in  its  nuclear  elements 
half  the  number  of  chromosomes  characteristic  of  the  body-cells.  The  number 
of  chromosomes  has,  therefore,  been  reduced,  hence  the  term  reduction  division. 
It  will  be  noted  that  the  actual  reduction  in  the  number  of  chromosomes  took 
place  when  they  were  first  formed  in  the  maturing  ovum,  while  the  spindle  or 


IMPREGNATION  OF  THE  OVUM.  49 

mitotic  figure  was  developing.  In  most  cases  a  second  polar  globule  is  produced 
a  short  time  after  the  first. 

When  this  occurs,  the  nuclear  remnants  in  the  ovum  do  not  reconstitute 
themselves  into  a  membranate  nucleus,  as  occurs  in  ordinary  cell  division,  but 
they  change  directly  into  a  second  spindle,  which  lies,  as  did  the  first,  within  the 
protoplasm  of  the  animal  pole.  This  second  spindle  occupies  an  oblique  posi- 
tion, or  may  even  be  parallel  with  the  surface.  But  it  also  soon  takes  up  a  radial 
position  and  produces  a  second  polar  globule  in  similar  manner  to  the  first.  The 
second  globule  is  usually  smaller  than  the  first. 

It  may  also  happen  that  the  first  polar  globule  may  itself  divide,  so  that 
three  polar  globules  appear. 

The  Formation  of  the  Female  Pro-nucleus. — The  nuclear  material,  which 
remains  in  the  main  ovum  after  the  separation  of  the  polar  globules,  is  known  as 
the  female  pro-nucleus.  The  nuclear  remnant  lies  close  to  the  animal  pole  and 
in  clear  protoplasm.  The  details  of  its  further  history  vary  according  to  the 
species  of  animal.  Three  tendencies  are  known  to  affect  the  pro-nucleus:  viz., 
to  move  toward  the  central  position  in  the  ovum,  to  unite  with  the  male  pro- 
nucleus  as  soon  as  that  is  formed  out  of  the  spermatozoa  which  enters  the  ovum, 
and  to  assume  the  character  of  a  membranate  nucleus.  As  the  time  of  the  for- 
mation of  the  male  pro-nucleus  is  variable,  the  other  tendencies  being  more 
constant,  the  exact  history  of  the  female  pro-nucleus  may  be  said  to  depend  prin- 
cipally on  the  appearance  of  the  male  pro-nucleus.  The  earlier  that  event,  the 
less  does  the  female  pro-nucleus  move  centrifugally  and  the  less  does  it  assume 
the  membranate  form.  Even  among  mammals  there  is  variation. 

Time  of  Maturation. — The  time  when  the  polar  globules  are  formed  varies 
according  to  the  animal,  and  may  be  before  or  after  the  egg-cell  leaves  the  ovary. 
In  placental  mammals,  maturation  always  begins,  so  far  as  known,  in  the  ovary, 
and  is  said 'in  some  cases  to  be  completed  there.  But  in  other  cases  it  is  cer- 
tainly completed  only  after  ovulation  or  when  the  ovum  has  passed  into  the 
Fallopian  tubes. 

Impregnation  of  the  Ovum. 

Impregnation  is  the  union  of  the  male  and  female  elements  to  form  a  single 
new  cell,  capable  of  initiating  by  its  own  division  a  rapid  succession  of  generations 
of  descendent  cells.  The  process  of  union  is  commonly  called  the  entrance  of  the 
spermatozoon  into  the  ovum.  The  new  cell  is  called  the  impregnated  or  ferti- 
lized ovum. 

In  all  multicellular  animals  impregnation  is  effected  by  three  successive 
steps:  (i)  The  bringing  together  of  the  male  and  female  elements;  (2)  the 

4 


50  THE  EARLY  DEVELOPMENT  OF  MAMMALS. 

entrance  of  the  spermatozoon  into  the  ovum  and  the  formation  of  the  male  pro- 
nucleus;   (3)  fusion  of  the  pro-nuclei  to  form  the  segmentation  nucleus. 

Meeting  of  the  Sexual  Elements. — In  all  amniota  the  seminal  fluid  is  trans- 
ferred from  the  male  to  the  female  passages  during  coitus,  and  spermatozoa  are 
thereafter,  in  mammals,  found  in  the  uterus.  In  default  of  actual  knowledge  it 
is  commonly  believed  that  the  spermatozoa  make  their  way  by  their  own  motions 
into  the  Fallopian  tubes.  The  ovum,  meanwhile  (probably,  in  mammals,  while 
completing  its  maturation),  travels  down  the  tube.  The  meeting-point,  or  site 
of  impregnation,  in  placental  mammals  is  about  one-third  way  down  from  the 
fimbria  to  the  uterus.  The  exact  spot  is  remarkably  constant  for  each  species. 
Nothing  is  known  by  direct  observation  as  to  the  site  of  impregnation  in  man, 
but  there  is  no  reason  to  suppose,  as  has  unfortunately  been  often  done,  that  the 
site  is  either  variable  or  essentially  different  from  that  in  other  mammals. 

The  Entrance  of  the  Spermatozoon  into  the  Ovum. — It  is  probable,  in  mam- 
mals at  least,  that  only  one  spermatozoon  enters  the  yolk  of  the  ovum  and  ac- 
complishes its  fertilization.  It  has  been  observed  in  those  animals  in  which,  as  in 
the  rabbit,  there  is  formed  a  more  or  less  considerable  space  between  the  yolk 
and  the  zona  radiata,  that  a  number  of  spermatozoa  appear  in  this  space,  but 
apparently  only  one  actually  fuses  with  the  substance  of  the  ovum.  The  manner 
in  which  additional  spermatozoa  are  excluded,  after  the  first  has  entered,  is  still 
under  discussion.  The  hypothesis  has  been  suggested  that  the  attractive  power 
of  the  ovum  is  annulled  or  weakened  by  the  formation  of  the  male  pro-nucleus 
from  the  spermatozoon  which  first  enters.  With  our  present  knowledge  the 
assumption  appears  unavoidable  that  the  ovum  exerts  a  specific  attraction  upon 
spermatozoa  of  the  same  animal  species.  Recent  authorities  incline  to  the  view 
that  this  attraction  is  of  a  chemical  nature,  for  it  has  been  observed  that  certain 
chemical  substances  may  attract  very  strongly  unicellular  organisms  capable  of 
free  locomotion.  The  phenomenon  is  called  chemotropism.  According  to  this 
interpretation,  the  attraction  of  the  ovum  for  the  spermatozoon  would  be  termed 
chemotropic. 

At  the  time  of  fertilization  the  ovum  in  the  Fallopian  tube  is  surrounded 
by  a  number  of  spermatozoa.  In  the  case  of  the  rabbit,  perhaps  by  a  hundred, 
more  or  less.  They  are  all,  or  nearly  all,  in  active  motion,  for  the  most  part 
pressing  their  heads  against  the  zona  radiata.  Several  of  them  may  make  their 
way  through  the  zona  into  the  interior.  According  to  Hensen,  only  those  sper- 
matozoa which  enter  the  zona  along  radial  lines  can  make  their  way  through. 
Those  which  take  oblique  courses  remain  caught  in  the  zona.  The  female  pro- 
nucleus  is  already  present,  either  formed  or  at  least  forming  as  a  membranate 
nucleus.  A  single  spermatozoa  makes  its  way  into  the  yolk  proper,  passing  a 


IMPREGNATION  OF  THE   OVUM.  51 

short  distance  into  the  interior.  It  is  uncertain  whether  the  whole  tail  of  the 
spermatozoon  enters  the  ovum  or  not.  In  some  of  the  lower  vertebrates  and  in 
other  animals,  it  appears  to  do  so.  It  is  probably  always  the  case-that  at  least 
the  main  piece  of  the  tail  enters  the  yolk.  The  tail  as  such  very  soon  disappears, 
while  the  head  of  the  spermatozoon  enlarges,  probably  by  the  imbibition  of 
fluid  from  the  surrounding  yolk.  The  head  of  the  spermatozoon  is  rich  in  chro- 
matin,  which  forms  a  series  of  irregujar  masses  as  the  head  enlarges,  producing 
a  network  appearance,  and  is  thus  converted  into  a  nucleus-like  body,  the 
male  pro-nucleus.  At  the  same  time  the  growing  head  surrounds  itself  in  some 
animals  by  a  membrane. 

We  now  have  a  cell  which  contains  two  nucleus-like  bodies,  one  derived 
from  the  head  of  the  spermatozoon  and  the  other  from  the  nucleus  of  the  egg- 
cell.  They  are  termed  respectively  the  male  and  female  pro-nucleus.  Each 
pro-nucleus,  when  it  first  appears,  is  small  and  gradually  enlarges,  probably  in 
both  cases  by  the  imbibition  of  fluid.  The  relative  size  of  the  two  pro-nuclei 
varies  considerably  in  different  species,  and  is  probably  a  secondary  and  rela- 
tively unimportant  relation.  The  proportion  between  the  two  probably  depends 
upon  the  time  when  the  male  pro-nucleus  is  formed.  If  the  spermatozoon  en- 
ters early  while  the  female  pro-nucleus  is  forming,  it  may  make  a  pro-nucleus  as 
large  as  that  from  the  egg-cell.  If,  on  the  other  hand,  the  spermatozoon  enters 
late,  the  female  pro-nucleus  enlarges,  acquires  a  start,  and  the  growing  male  pro- 
nucleus  is,  therefore,  smaller. 

Concerning  the  fate  of  the  middle  piece  of  the  spermatozoon  and  its  share 
in  the  fertilization  in  the  ovum  of  mammals,  we  possess  no  satisfactory  informa- 
tion. It  has  been  shown,  however,  in  other  animals  that  this  middle  piece  pro- 
duces a  centrosome,  and  the  only  centrosome  which  appears  in  the  fertilized 
ovum.  The  theory  has  been  advanced  that  the  ovum,  after  its  maturation,  has 
no  centrosome,  that  a  centrosome  is  always  brought  into  the  ovum  by  the  sper- 
matozoon in  the  manner  just  indicated.  If  we  regard  the  centrosome  as  a  per- 
manent cell  element,  then  we  must  further  interpret  the  addition  of  the  male 
centrosome  as  one  of  the  most  important  phenomena  of  fertilization.  Whether 
this  hypothesis  is  correct  or  not,  we  are  unable  at  present  to  decide. 

Astral  figures  play  a  conspicuous  part  in  the  phenomenon  of  fertilization  in 
many  animals.  Astral  figures  are  produced  in  the  protoplasm  of  the  ovum  by 
its  assuming  a  special  radiating  structure.  Astral  figures  may  appear  around 
both  the  male  and  female  pro-nuclei  (Fig.  3).  In  other  cases  the  astral  figure 
arises  only  in  association  with  the  head  of  the  spermatozoon  or  male  pro-nucleus. 
In  mammals,  so  far  as  known,  no  astral  figures  are  developed  about  either  of  the 
pro-nuclei.  There  is  a  clear  space  in  the  protoplasm  around  each  nucleus,  and 


52 


THE  EARLY  DEVELOPMENT  OF  MAMMALS. 


such  a  clear  space  has  often  been  noted  also  when  the  astral  figure  is  present.  It 
may  possibly  be  interpreted  as  a  rudimentary  aster  or  center  of  astral  formation. 
The  two  pro-nuclei  usually  lie  at  some  distance  from  one  another.  As  soon 
as  they  are  formed,  or  perhaps  when  they  are  fully  differentiated,  they  tend  to 
move  toward  one  another  and  toward  the  center  of  the  ovum.  Concerning  the 
path  of  the  male  pro-nucleus  we  possess  interesting  information  from  the  study 
of  the  ova  of  the  frog  and  axolotl.  In  these  ova  the  spermatozoon  leaves  a  trail 
of  pigment,  which  consists  of  two  limbs,  one  passing  through  the  cortical  layer  of 
the  ovum  nearly  perpendicular  to  the  surface,  and  the  other  forming  an  angle 
with  the  first  and  leading  directly  to  the  female  pro-nucleus.  The  female  pro- 
nucleus  tends  always  to  move  toward  a  central  position.  The  force  which  draws 


FIG.  3.— OVUM  OF  A  WORM  (SAGITTA)  WITH  Two 
PRO-NUCLEI.  AROUND  EACH  PRO-NUCLEUS  is 
SHOWN  THE  ASTER. — (After  O.  Hertwig.} 


FIG.  4. — OVUM  OF  A  RABBIT,  SKVENTEEN  HOURS 
AFTER  COITUS,  WITH  THE  PRO-NUCLEI  ABOUT 
TO  CONJUGATE. 

p.g,  Polar  globules. — (After  Rein.} 


the  pro-nuclei  together  is  unknown.     The  hypothesis  that  this  force  is  chemo- 
tropic  has  met  with  favor. 

The  fusion  of  the  pro-nuclei. — In  the  rabbit,  as  probably  in  all  mammals, 
both  pro-nuclei  lie  at  first  eccentrically,  but  both  move  toward  each  other  and 
toward  the  center,  meeting  when  the  central  position  is  attained.  As  they  near 
one  another,  both  pro-nuclei  perform  active  amoeboid  movements.  After  they 
meet  they  still  continue  their  amoeboid  movements,  and  travel  together  to  the 
center  of  the  ovum  (Fig.  4).  One  of  the  pro-nuclei  assumes  a  crescent  shape  and 
embraces  the  other.  In  the  mouse  the  history  is  similar.  After  the  two  pro- 
nuclei  in  this  animal  have  met,  they  remain  side  by  side,  they  are  without  mem- 
branes, the  chromatin  threads  become  distinct  and  draw  closer  together.  Between 
them  appears  first  a  small  spot  or  centrosome  with  a  few  radiating  lines  between 
it  (Fig.  5).  From  the  centrosome  arises  a  spindle  of  achromatic  threads  (Fig.  6). 


IMPREGNATION  OF  THE  OVUM. 


53 


The  chromatin  of  each  pro-nucleus  now  forms  a  group  of  well-defined,  elongated, 
somewhat  crooked  chromosomes.  The  two  groups  of  chromosomes  are  quite 
distinct,  and  are  separated  from  one  another  by  the  intervening  spindle.  The 
spindle  continues  to  grow,  and  the  chromosomes  of  the  male  pro-nucleus  on  the 
one  side,  and  the  female  pro-nucleus  on  the  other,  attach  themselves  to  the 
equatorial  region  of  the  spindle  (Fig.  7).  The  spindle  continues  to  grow;  the 
chromosomes  become  V-shaped  and  arrange  themselves  as  the  so-called  equa- 
torial plate,  in  which  the  chromosomes  of  the  two  pro-nuclei  can  no  longer  be 
distinguished  from  one  another.  At  each  end  of  the  spindle  is  a  distinct  centro- 
some  with  a  very  faint,  small  astral  radiation  in  the  neighboring  protoplasm.  This 
spindle  is  the  beginning  of  the  division  of  what  we  may  call  the  segmentation 


-&<• 


;$;:•;  •     >  ;•:;.;,•: Xl 


m#$&?:-$ 


Fir,.  5. — OVUM  OF  WHITE  MOUSE.       FIG.  6. — OVUM  OF  WHITE  MOUSE.        FIG  7. — OVUM  OF  WHITE  MOUSE. 


BEGINNING  OF  THE  CONJUGA- 
TION OF  THE  PRO-NUCLEI.  X 
1500  diams. — (After  Sobotta.} 


CONJUGATION    OF    THE    PRO- 
NUCLEI,    AND  FORMATION     OF 

THE    SEGMENTATION    SPINDLE. 
X  1500  diams. — (After  Sobotta.} 


FIRST  SEGMENTATION  SPINDLE 
WITH  THE  CHROMOSOMES  OF 
THE  PRO-NUCLEI  STILL  FORM- 
ING Two  DISTINCT  GROUPS. 
X  I 500  diams.  — (After  Sobotta. } 


nucleus.  In  the  mouse  the  two  pro-nuclei  do  not  actually  fuse  into  a  single 
nucleus  before  the  formation  of  the  spindle,  which  initiates  the  first  division  of 
the  fertilized  ovum,  so  that,  strictly  speaking,  there  is  no  fusion  of  the  pro-nuclei 
to  make  a  segmentation  nucleus.  There  is,  nevertheless,  a  true  fusion  of  the 
pro-nuclei  accomplished,  although  it  is  somewhat  masked  by  the  early  com- 
mencement of  the  first  segmentation  spindle,  which  develops  at  the  same  time 
that  the  fusion  of  the  pro-nuclei  is  being  completed.  Whether  the  processes  as 
described  in  the  mouse  are  typical  for  mammals  we  do  not  know,  the  white  mouse 
being  the  only  species  in  which  the  process  has  been  followed  in  detail.  The 
student  will  find  some  additional  information  in  the  practical  part. 

It  is  now  believed  that  the  pro-nuclei  never  unite  to  form  a  distinct  mem- 


54  THE  EARLY  DEVELOPMENT  OF  MAMMALS. 

branate  nucleus,  the  so-called  segmentation  nucleus  of  earlier  writers,  but  that 
the  fusion  always  takes  place  during  the  absence  of  the  membranes  of  the  pro- 
nuclei  by  the  mingling  of  their  contents.  The  time  of  mingling,  however,  varies 
as  regards  the  formation  of  the  chromosomes.  It  may  take  place  before  or  after 
the  chromosomes  are  developed.  When,  as  in  the  mouse,  the  chromosomes  ap- 
pear as  two  distinct  groups,  it  is  possible  sometimes  to  determine  their  number. 
In  the  mouse  counting  is  difficult,  but  there  seems  little  doubt  that  each  pro- 
nucleus  forms  twelve  chromosomes.  Hence  it  results  that  there  are  twenty-four 
chromosomes  in  the  segmentation  spindle.  This  number,  twenty-four,  so  far  as 
has  been  determined,  is  the  number  which  appears  during  later  stages  of  seg- 
mentation and  in  all  subsequent  cell  divisions  of  this  animal.  It  is  believed  to 
be  a  general  law  that  the  male  and  female  prornuclei  each  contribute  the  same 
number  of  chromosomes  to  the  segmentation  spindle.  This  number  is  identical 
with  the  number  which  appears  during  the  reduction  divisions  which  lead  to  the 
maturation  of  the  ovum,  on  the  one  hand,  and  the  development  of  the  sperma- 
tozoon on  the  other;  and,  further,  the  number  is  one-half  the  number  of  chromo- 
somes which  appear  during  ordinary  cell  divisions  of  the  species.  The  most 
thorough  study  of  the  phenomenon  which  has  yet  been  made  is  that  by  a  succes- 
sion of  able  investigators  upon  the  large  nematode  Ascaris  megalocephala.  An 
admirable  summary  of  the  process  of  fertilization  in  Ascaris  has  been  given  by 
Oscar  Hertwig.* 

Segmentation  of  the  Ovum. 

Shortly  after  the  entrance  of  the  spermatozoon  into  the  ovum  the  segmenta- 
tion spindle  is  developed  by  the  union  of  the  pro-nuclei,  as  described  in  the  pre- 
vious section.  This  spindle  leads  to  a  division  of  the  ovum  into  two  cells.  These 
cells  further  rapidly  divide.  As  stated  on  page  26,  these  early  cell  divisions  are 
called  the  segmentation  of  the  ovum. 

The  position  of  the  segmentation  spindle  is  always  eccentric,  and  appears  to 
be  approximately,  if  not  exactly,  the  same  as  that  of  the  egg-cell  nucleus  before 
maturation.  The  axis  of  the  spindle  varies  greatly  in  its  direction.  It  is  some- 
times at  right  angles  to  the  radius  of  the  ovum,  which  passes  through  the  polar 
globules,  but  it  is  more  usually  oblique  to  this  radius.  It  was  at  one  time  thought 
that  the  plane  of  division  was  always  at  right  angles  to  the  radius  of  the  extrusion 
of  the  polar  globules,  but  this  view  cannot  be  upheld.  After  the  ovum  has 


*  "  Lehrbuch  der  Entwicklungsgeschichte,"  sixth  edition,  1898.  The  large  Ascaris  is  a  particularly 
favorable  object.  The  student  who  wishes  to  pursue  the  practical  study  of  impregnation  further  should  select 
this  form.  Material  suitably  preserved  may  be  obtained  from  the  Preparator  of  the  Department  of  Zoology, 
University  of  Pennsylvania. 


SEGMENTATION  OF  THE  OVUM. 


55 


divided  into  two  cells,  the  polar  globules  lie  in  the  angle  between  these  two  cells 
(Fig.  8),  because  there  the  globules  find  room.  It  is  to  be  noted  that  the  glo- 
bules accommodate  themselves  to  the  segmentation  spheres,  and  that  the  forma- 
tion of  the  spheres  is  not  accommodated  to  the  original  position  of  the 
globules.  « 

The  degree  of  the  eccentricity  of  the  segmentation  spindle  varies  in  different 
ova,  chiefly  according  to  the  amount  of  yolk ;  the  greater  the  quantity  of  yolk 
in  the  ovum,  the  more  marked  is  the  eccentricity. 

The  actual  first  cell  division  (first  cleavage  or  first  segmentation)  of  a  mam- 
malian ovum  has  never  been  followed  by  direct  observation,  the  practical  diffi- 
culties not  having  hitherto  been  successfully  overcome.  Various  phases  of  the 


FIG.    8. — OVUM   OF    A  RABBIT  OF    TWENTY-FOUR        FIG.  9.— OVUM  OF  A   SNAIL   (LIMAX  CAMPESTRIS) 
HOURS.  DURING  THE  FIRST  CLEAVAGE.     THE  ENVEL- 

The  first  cleavage  has  been  completed  ;  the  two  cells  OPES    OF    THE    OVUM    ARE    NOT    DRAWN    IN 

(segmentation  spheres)  are  appressed;  above  the  X  200  diams. — (After  E.  L.  Mark.} 

cells  lie  the  polar  globules  ;  numerous  spermato- 
zoa lie  in  and  within  the  zona  pellucida. — (After 
Coste. ) 

division  have,  however,  been  seen  and  the  internal  changes  have  been  studied  by 
means  of  sections.  It  accordingly  seems  expedient  to  interpolate  the  following 
account  of  the  external  appearances  of  the  first  segmentation  in  the  living  ovum 
of  the  snail,  Limax  campestris.  The  eggs  of  this  animal,  by  their  size  and  in 
their  mode  of  segmentation,  have  a  certain  resemblance  with  mammalian  ova. 
The  following  description  is  taken  from  the  account  by  E.  L.  Mark,  published  in 
1 88 1 ;  it  is  nearly  in  his  own  words  : 

In  Limax,  after  impregnation,  the  region  of  the  segmentation  nucleus 
remains  more  clear,  but  all  that  can  be  distinguished  is  a  more  or  less  circular, 
ill-defined  area,  which  is  less  opaque  than  the  surrounding  portions  of  the  vitel- 
lus.  After  a  few  moments  this  area  grows  less  distinct.  It  finally  appears 


56 


THE  EARLY  DEVELOPMENT  OF  MAMMALS. 


elongated.  Very  soon  this  lengthening  results  in  two  light  spots,  which  are  in- 
conspicuous at  first,  but  which  increase  in  size  and  distinctness,  and  presently 
become  oval.  If  the  outline  of  the  egg  be  carefully  watched,  it  is  now  seen  to 
lengthen  gradually  in  a  direction  corresponding  to  the  line  which  joins  the  spots. 
As  the  latter  enlarges  the  lengthening  of  the  ovum  increases,  though  not  very 
conspicuously.  Soon  a  slight  flattening  of  the  surface  appears  just  under  the 
polar  globules;  the  flattening  changes  to  a  depression  (Fig.  9),  which  grows 
deeper  and  becomes  angular.  A  little  later  the  furrow  is  seen  to  have  extended 
around  on  the  sides  of  the  yolk  as  a  shallow  depression,  reaching  something  more 
than  half-way  toward  the  vegetable  or  inferior  pole,  and  in  four  or  five  minutes 


FIG.  10. — OVUM  OF  WHITE  MOUSE.  FIRST  SEG- 
MENTATION SPINDLE  WITH  EQUATORIAL  PLATE 
OF  CHROMOSOMES.  X  I5°°  diams. — (After 
Sobotta. ) 


FIG.  ii. — OVUM  OF  WHITE  MOUSE.  FIRST  SEG- 
MENTATION SPINDLE. 

The  chromosomes  have  divided  and  have  migrated 
toward  the  poles  of  the  spindle,  forming  two 
groups.  X  1500  diams. — (After  Sobotta.} 


after  its  appearance  the  depression  extends  completely  around  the  yolk.  This 
annular  constriction  now  deepens  on  all  sides,  but  most  rapidly  at  the  animal 
pole;  as  it  deepens  it  becomes  narrower,  almost  a  fissure.  By  the  further  deep- 
ening of  the  constriction  on  all  sides  there  are  formed  two  equal  masses  connected 
by  only  a  slender  thread  of  protoplasm,  situated  nearer  the  vegetative  than  the 
animal  pole,  and  which  soon  becomes  more  attenuated  and  finally  parts.  The 
first  cleavage  is  now  accomplished.  Both  segments  undergo  changes  of  form; 
they  approach  and  flatten  out  against  each  other,  and  after  a  certain  time  them- 
selves divide. 

The  Internal  Changes  in  the  Mammalian  Ovum. — At  the  close  of  impregna- 
tion the  segmentation  spindle  is  completely  formed  (Fig.  10).     The  chromosomes 


SEGMENTATION  OE  THE  OVUM. 


57 


of  the  equatorial  plate  now  divide  probably  by  splitting  longitudinally  so  that 
the  number  of  chromosomes  is  doubled.  During  the  splitting  the  chromosomes 
tend  to  draw  apart  from  one  another.  At  the  same  time  the  spindle,  without 
changing  its  length,  becomes  somewhat  narrower.  The  chromosomes  now  move 
apart  from  the  equator  toward  the  two  poles,  forming  two  groups,  each  group 
containing  half  of  the  total  number  of  chromosomes  (Fig.  1 1),  and  at  the  same 
time  the  whole  ovum  becomes  somewhat  elongated  in  the  direction  correspond- 
ing with  the  axis  of  the  spindle.  The  chromatin  granules  accumulate  at  the  two 
poles  of  the  spindle.  The  achromatic  threads  between  the  poles  break  through. 
Then  the  actual  cleavage  of  the  elongated  ovum  into  two  cells  becomes  marked 
in  the  protoplasm,  and  the  line  of  separation  of  the  two  cells  passes  through  the 
equator  of  the  spindle.  The  accumulated  granules  of  chromatin  then  reconsti- 
tute the  resting  membranate  nucleus  (Fig.  12).  In  brief,  the  segmentation  of 


FIG.  12.— OVA  OF  WHITE  MOUSE  WITH- Two  SEGMENTATION  SPHERES  OR  CELLS. 

A,  Telophase  of  the  division  ;  the  chromosomes  are  reconstituting  the  nucleus.  £,  Membranate  nucleus  recon- 
stituted. /,  First  cell  of  segmentation.  nu,  Nucleus,  p.g,  Polar  globules.  Z,  Zona  pellucida.  X  5°° 
diams. — (After  Sobotta.) 

the  ovum  is  a  typical  indirect  or  mitotic  cell  division.  In  the  mouse  the  first 
cleavage  is  completed  about  twenty-six  hours  after  the  coitus.  The  second 
cleavage  is  not  completed  until  twenty-four  hours  later.  When  first  formed,  two 
segmentation  spheres  are  oval  and  entirely  separated  from  one  another,  but 
subsequently  they  flatten  against  one  another  and  become  appressed,  a  phe- 
nomenon of  which  we  have  no  explanation.  In  most  mammals  which  have  been 
studied  there  is  more  or  less  space  between  the  segmenting  ovum  and  the  zona 
(see  Fig.  8),  but  in  the  mouse  this  space  is  reduced  to  a  minimum  and  the  zona 
is  often  stretched  into  irregular  forms  during  the  changes  of  the  ovum. 

The  succeeding  cleavages  of  segmentation  need  to  be  followed  out  in  greater 
detail  than  yet  recorded.  In  many  cases  there  appear  to  be  three  cells  in  the 
next  stage,  because  one  of  the  two  primitive  segmentation  spheres  divides  sooner 
than  the  other.  The  more  commonly  received  view  is  that  four  cells  are  pro- 


58 


THE  EARLY  DEVELOPMENT  OF  MAMMALS. 


duced  next,  but  it  may  very  well  be  that  there  is  really  a  three-cell  stage  preced- 
ing the  four-cell  stage  of  which  two  figures  are  presented.  The  first  of  these 
(Fig.  13)  represents  the  four-cell  stage  of  the  ovum  of  a  bat,  and  the  second  (Fig. 
14)  represents  the  four-cell  stage  of  the  ovum  of  the  Virginian  opossum.  That 
of  the  bat  resembles  the  picture  which  we  obtained  from  a  number  of  animals, 
such  as  the  rabbit,  the  guinea-pig,  the  dog,  and  others.  That  of  the  opossum 
differs  so  much  from  anything  known  in  other  mammals  that  it  may  be  ques- 
tioned whether  it  is  entirely  normal.  In  the  mouse  the  zona  is  much  thinner 
and  assumes  an  irregular  form,  adapting  itself  to  the  pressure  of  the  single 
spheres. 

After  the  four-cell  stage,  the  segmentation  proceeds  apparently  with  con- 


X300 


FIG.  13. — OVUM  OF  A  BAT  (VESPERTILIO  MURINA) 
WITH  FOUR  SEGMENTATION  SPHERES. — (After 
van  Beneden  and  Julin.] 


FIG.   14. — OVUM  OF  A   VIRGINIAN   OPOSSUM,  WITH 
FOUR  SEGMENTS. — (After  Emil  Selenka. ) 


siderable  irregularity,  but  we  are  soon  able  to  see  that  the  cells  are  grouping 
themselves  into  an  uninterrupted  external  layer  and  an  internal  accumulation  of 
cells.  The  outer  layer  is  in  contact,  or  nearly  in  contact,  with  the  zona  radiata, 
and  may,  therefore,  be  termed  the  subzonal  layer  (Fig.  16,  s.z).  The  inner 
accumulation  of  cells  is  designated  as  the  inner  mass,  i.m.  Fig.  15  represents  a 
rabbit  ovum  of  about  seventy  hours,  according  to  the  observations  of  van  Bene- 
den. He  represents  the  subzonal  layer,  EC,  as  interrupted  at  one  point,  where  one 
of  the  cells  of  the  inner  mass,  i.m,  is  exposed.  It  is  probable,  however,  that  van 
Beneden  is  in  error  in  regard  to  this,  and  that  the  subzonal  layer  is  really  con- 
tinuous. In  the  next  stage  (Fig.  16)  we  find  that  the  ovum  has  become  larger 
by  the  appearance  of  a  cavity  in  its  interior.  This  cavity  appears  between  the 


SEGMENTATION  OF  THE   OVUM. 


59 


inner  mass,  i.m,  and  the  subzonal  layer,  but  at  one  side  the  inner  mass  remains 
adherent  to  and  closely  connected  with  the  subzonal  layer.  We  now  have 
reached  the  stage  in  which  the  developing  ovum  may  be  designated  as  the  blasto- 
dermic  vesicle. 

As  to  the  interpretation  of  the  parts,  it  is  probable  that  the  subzonal  layer 
is  the  ectoderm,  and  that  the  inner  mass  is  the  entoderm.  At  the  stage  we  have 
now  reached  the  blastodermic  vesicle  has  a  large  part  of  its  walls  formed  by  the 
subzonal  layer  only,  so  that  we  call  this  the  stage  of  the  one-layered  blastodermic 
vesicle. 

Arrival  in  the  Uterus. — During  the  stages  described  the  ovum  travels  along 
the  Fallopian  tube  and  reaches  the  uterus  in  an  early  phase  of  the  stage  which  we 


um. 


FIG.  15. — RABBIT'S    OVUM    OF    ABOUT    SEVENTY 

HOURS. 

EC,  Outer  layer,      i.m,  Inner  mass  of  cells.      Z,  Zona 
pellucida. — (After  E.  van  Beneden.} 


FIG.    16. — YOUNG    BLASTODERMIC    VESICLE   OF   A 

MOLE. 

i.///,    Inner   mass   of  cells,     s.z,    Outer   or  subzonal 
layer,      z,  Zona  pellucida. — (After  W.  ffeafe.) 


designate  as  the  blastodermic  vesicle.  The  transit  requires  about  eighty  hours  in 
the  mouse,  about  five  days  in  the  opossum,  four  days  in  the  rabbit,  and  eight  to 
ten  days  in  the  dog.  The  time  necessary  in  man  is  unknown.  It  may  be  sup- 
posed to  be  about  one  week. 

Pro-chorion. — The  ovum  in  many  mammals  becomes  surrounded  by  a  gelat- 
inous covering,  which  is  secreted  by  the  glands  of  the  uterus.  It  may  be  com- 
pared with  the  white  of  the  bird's  egg.  In  the  rabbit  this  envelope  becomes 
enormously  thick  about  the  blastodermic  vesicle  and  in  other  rodents  is  volu- 
minous. In  the  dog  it  is  less  developed,  but  presents  the  further  peculiarity 
that  the  secretion  in  the  tubular  glands  may  be  hardened  in  connection  with  the 


60  THE  EARLY  DEVELOPMENT  OF  MAMMALS. 

envelope  itself,  which,  therefore,  appaars,  when  the  ovum  is  removed  from  the 
uterus,  to  be  studded  over  with  fine  threads  resembling  villi.  The  gelatinous 
envelope  has  been  termed  by  Hensen  the  pro-chorion.  The  thread-like  pro- 
jections seen  in  the  dog  were  taken  by  Bischoff  for  true  villi,  and  they  have  some- 
times been  referred  to  as  the  pro-chorionic  villi.  The  term  pro-chorion  has  been 
applied  to  other  structures,  as,  for  instance,  to  the  subzonal  layer  of  the  blasto- 
dermic  vesicle.  The  student  needs  to  be  warned  against  confusing  the  term 
pro-chorion  in  its  various  applications. 

The  Blastodermic  Vesicle. 

The  blastodermic  vesicle  always  consists  at  first  of  the  subzonal  layer  and 
an  inner  cell  mass  attached  at  one  point  to  the  subzonal  layer,  and  has  a  cavity 
between  the  inner  mass  and  the  subzonal  layer;  the  vesicle  itself  is  always 
enclosed  in  the  zona  radiata.  The  variations  offered  in  different  mammals  are 
so  great  that  a  description  less  general  than  that  given  would  hardly  be  appli- 
cable, even  to  the  placental  mammals. 

The  next  step  in  development  is  the  production  of  a  complete  second  layer 
out  of  the  cells  of  the  inner  mass.  This  layer  extends  completely  around  the 
vesicle  and  lies  close  against  the  subzonal  layer,  and  encloses  the  main  cavity 
of  the  vesicle.  The  way  in  which  this  inner  vesicular  layer  is  developed  varies 
greatly.  In  the  hedgehog  it  appears  very  precociously,  while  the  blastodermic 
vesicle  is  very  small,  and  afterward  it  expands  rapidly,  while  the  vesicle  as  a 
whole  is  growing.  In  the  rabbit  and  in  the  mole  it  is  formed  much  later,  and  the 
one-layered  vesicle  expands  to  a  considerable  diameter  before  the  inner  mass 
begins  to  spread  out.  The  striking  changes  through  which  the  inner  mass  passes 
in  the  mole  are  illustrated  in  Fig.  17.  It  forms  at  first  a  small  globe,  A.  The 
inner  mass  subsequently  flattens  out,  becoming  lens-shaped,  thinner,  and  larger 
in  area,  B.  It  continues  spreading  laterally  and  separates  into  three  layers. 
The  two  outer  layers  enter  into  the  formation  of  the  true  ectoderm,  C.  In  the 
rabbit,  and  perhaps  in  the  mole,  the  outermost  layer  is  temporary  only  in  exist- 
ence. In  some  rodents  it  acquires  a  very  great  development  and  leads  to  the 
curious  phenomenon  known  as  the  inversion  of  the  germ-layers.  The  innermost 
of  the  layers  grows  at  its  edges,  and  its  cells  spread  out  gradually  further  and 
further  under  the  subzonal  layer  until  they  extend  completely  around  the  vesi- 
cle and  form,  by  meeting  at  the  opposite  pole  of  the  ovum,  a  closed  vesicle. 
Very  similar  is  the  process  in  the  rabbit.  The  cells  at  the  expanding  edge  of  the 
inner  layer  are  found  to  spread  rapidly,  so  that  during  the  expansion  they  are 
more  or  less  widely  separated  from  one  another.  But  they  continue  their  expan- 
sion and  multiplication  until  they  form  a  complete  inner  epithelial  layer. 


THE  BLASTODERMIC  VESICLE. 


61 


The  point  where  the  inner  mass  and  the  subzonal  layers  are  connected  with    - 
one  another  marks  the  site  of  the  future  embryonic  area. 

The  blastodermic  vesicle  grows  rapidly  in  size,  partly  by  the  multiplication 
of  its  cells,  partly  by  their  becoming  flattened  out  so  as  to  cover  a  larger  surface. 
The  interior  of  the  vesicle  is  filled  with  fluid.  As  the  vesicle  grows  the  fluid  in- 
creases in  amount,  and  is  presumably 
derived  by  the  ovum  from  the  walls  of 
the  uterus.  It  is  under  pressure  within 
the  vesicle,  as  is  shown  by  the  manner 
in  which  it  spurts  out  if  the  vesicle  is 
broken.  Nothing  exact  as  to  the  com- 
position of  this  fluid  is  known,  though 
we  may  suppose  it  to  resemble  more  or 
less  the  serous  fluids  of  the  adult  body. 
The  size  and  form  of  the  vesicle  offer 
characteristic  variations  in  mammals. 
It  starts  as  a  more  or  less  nearly 
spherical  body.  In  the  rabbit  it  as- 
sumes an  oval  shape,  and  by  the  seventh 
day  measures  about  4.0  mm.,  and  soon 
thereafter  becomes  attached  to  the  wall 
of  the  uterus.  In  the  hedgehog,  the 
guinea-pig,  and  the  mouse  the  ovum, 
while  very  small  and  more  or  less 
rounded  in  form,  becomes  imbedded  in 
uterine  tissue  and  develops  into  a 
special  shape  in  adaptation  to  its  new 
situation.  In  the  ungulates  the  vesicle 
grows  enormously,  becoming  a  very 
long  and  slender  sack.  Thus,  for  ex- 
ample, in  the  sheep  it  may  measure 
on  the  fourteenth  day  not  less  than  50 
cm.  in  length. 

Another  respect  in  which  the  blastodermic  vesicles  differ  greatly  from  one 
another  in  various  mammals  is  in  regard  to  the  early  development  of  the  sub- 
zonal  layer,  or,  as  we  may  call  it,  the  ectoderm.  In  many  cases  the  entire  layer 
undergoes  a  precocious  development,  its  cells  multiply  very  rapidly,  so  that  the 
layer  becomes  several  cells  thick.  This  thickened  layer  is  known  as  the  tropho- 
blast.  In  other  placental  mammals  this  thickening  is  confined  to  a  limited  area 
of  the  ectoderm.  For  further  description  see  Trophoblast. 


sec.c- 


FIG.  17. — SECTIONS   THROUGH   THE  INNER   MASS 

OF   BLASTODERMIC  VESICLES   OF  THE   MOLE 

AT  THREE  SUCCESSIVE  STAGES. 
EC,  Outer  or  subzonal  layer,     z,  jc,  Zona  pellucid  a. 

t.m,    Inner  mass    of   cells.       hy,    Entoderm. — 

(After  W.  He  ape.) 


62 


THE  EARLY  DEVELOPMENT  OF  MAMMALS. 


*   The  Embryonic  Shield. 

Sooner  or  later  in  the  early  history  of  every  blastodermic  vesicle,  and  always 
as  the  first  indication  of  the  development  of  the  embryo  proper,  there  appears  a 
thickening  of  a  small  oval  area  of  the  outer  layer  of  the  blastodermic  vesicle. 
This  thickening  is  known  as  the  embryonic  shield.  In  the  fresh  specimen  it 
marks  itself  by  the  greater  opacity  which  it  causes  in  the  walls  of  the  ovum 
where  it  lies.  It  is  produced  always  at  the  point  where  the  inner  cell  mass  was 
originally  attached  to  the  subzonal  layer.  In  those  cases  where  the  thickening 
of  the  ectoderm  to  form  the  trophoblast  extends  over  the  entire  blastodermic 
vesicle,  it  is  very  difficult  to  follow  the  early  history  of  the  embryonic  shield.  In 
other  cases,  however,  where  the  trophoblast  occupies  a  special  restricted  area, 
the  history  of  the  embryonic  shield  may  be  more  readily  followed.  The  animals 
in  which  it  has  hitherto  been  chiefly  studied  are  the  rabbit,  dog,  cat,  and  sheep. 
In  all  of  these  the  embryonic  shield  is  simply  a  thickening  of  the  outer  layer  (Fig. 


FIG.  18. — TRANSVERSE  SECTION  THROUGH  THE   EMBRYONIC    SHIELD  JDF   THE   BLASTODERMIC  VESICLE  OK 
A  DOG  OF  ELEVEN  OR  FIFTEEN  DAYS  (PRECISE  AGE  UNKNOWN). 
O.L,  Outer  layer.     Ent,  Entoderm.     X  2O°  diams. — (After  Sonnet.} 


1 8).  The  embryonic  shield  is  at  first  small,  but  it  rapidly  expands  and  assumes 
a  rounded  or  oval  form.  There  next  appears,  in  a  more  or  less  central  position 
in  the  shield,  a  small,  darker  spot,  which  marks  what  is  known  as  the  primitive 
knot,  a  peculiarity  of  which  is  that  it  corresponds  to  an  intimate  union  of  the 
cells  of  the  inner  with  those  of  the  outer  layer  of  the  blastodermic  vesicle.  Soon 
a  linear  shadow  becomes  visible  extending  from  the  primitive  knot  toward  a 
point  at  the  periphery  of  the  embryonic  shield — Fig.  19,  p.  s,  which  represents 
the  embryonic  shield  of  a  dog  at  about  two  weeks.  The  shadow  from  the  primi- 
tive knot  is  termed  the  primitive  streak,  and  it  very  soon  becomes  further  charac- 
terized by  the  formation  of  a  fine  groove  caused  by  a  depression  in  the  outer 
layer  of  cells.  This  is  known  as  the  primitive  groove,  and  has  been  observed  in 
all  amniote  embryos.  Its  exact  significance  has  never  been  satisfactorily  ascer- 
tained, and  its  interpretation  is  still  a  matter  of  scientific  discussion.  A  trans- 
verse section  through  the  primitive  streak  of  a  vesicle  of  a  common  European 


ORIGIN  OF  THE  MESODERM. 


G3 


mole  is  shown  in  Fig.  23.  At  about  the  time  the  primitive  streak  appears  the 
embryonic  shield  becomes  oval  in  form.  In  those  animals,  such  as  the  carnivora 
and  ungulates,  which  have  a  large  elongated  blastodermic  vesicle,  we  find  that 
the  long  axis  of  the  embryonic  shield  is  nearly  at  right  angles  to  the  long  axis  of 


SA. 


FIG.  19. — SURFACE  VIEW  OF  THE  EMBRYONIC  SHIELD  OF  THE  BLASTODERMIC  VESICLE  OF  A  DOG  OF 
THIRTEEN  TO  FIFTEEN  DAYS  (PRECISE  AGE  UNKNOWN). 

The  specimen  had  been  preserved  with  sublimate  and  stained  with  borax-carmin.     SA,  Embryonic  shield.     Kn, 
Hensen's  knot,     p.s,  Primitive  streak.     X  IO°  diams. — (After  Bonnet.) 


the  vesicle.     The  size  of  the  shield  is  about  the  same  in  all  mammals  which  have 
been  heretofore  studied. 


Origin  of  the  Mesoderm. 

The  development  of  the  primitive  streak  and  groove  is  accompanied  by  the 
appearance  of  the  third  or  middle  germ-layer,  the  mesoderm  (Fig.  20,  mes).  As 
shown  in  the  section  there  figured,  the  three  germ-layers  are  fused  together  under- 
neath the  primitive  groove,  and  are  there  thicker  than  elsewhere.  As  we  pass 
laterally  from  the  groove,  the  ectoderm  and  mesoderm  both  become  thinner  and 
are  distinctly  separated  from  one  another.  The  entoderm  consists  of  a  single 


64 


THE  EARLY  DEVELOPMENT  OF  MAMMALS. 


thin  layer  of  cells  very  closely  connected  with  the  mesoderm.  The  mesoderm 
occupies  at  first  only  a  small  area  in  the  immediate  neighborhood  of  the  primitive 
streak.  It  grows  rapidly,  so  that  its  edge  extends  further  and  further  over  the 
blastodermic  vesicle.  The  mesoderm  is  to  be  regarded  as  the  product  of  the 
entoderm.  Its  exact  origin  in  mammals  has  not  yet  been  adequately  traced. 
We  know,  however,  that  in  birds,  reptiles,  and  elasmobranchs  the  cells  of  the 
inner  layer  multiply  rapidly,  so  that  the  inner  layer  becomes  more  than  one 


FIG.  20. — CENTRAL  PORTION  OF  A  SHEEP'S  BLASTO- 
DERMIC VESICLE  OF  TWELVE  TO  THIRTEEN 
DAYS. 

S/i,  Embryonic  shield.  Kn,  Hensen's  knot,  vies, 
Shadow  caused  by  mesoderm  developing  around 
the  shield.  X  34  diams. — (After  Bonnet.} 


FIG.  21. — BLASTODERMIC  VESICLE  OF  A  RABBIT 
OF  SEVEN  DAYS.  PORTION  OF  THE  MESO- 
DERM OF  THE  AREA  OPACA. — (After  Kdlliker.} 


cell  thick.  The  upper  cells  soon  split  off  from  the  lower  and  thus  form  them- 
selves into  the  middle  germ-layer.  The  mesoderm  therefore  is  said  to  be 
formed  by  delamination.  It  seems  probable  that  in  mammals  the  process  is 
the  same. 

It  may  be  mentioned  that,  according  to  Bonnet,  the  development  of  the 
mesoderm  is  not  quite  as  above  described.  It  can  be  first  distinguished  at  the 
stage  when  the  primitive  knot  has  appeared,  and  before  the  primitive  streak  is 
developed.  In  the  fresh  specimen  it  is  seen  as  a  slight  turbidity  of  the  vesicular 


THE  PRIMITIVE  AXIS.  65 

walls  just  outside  the  edge  of  the  shield  (Fig.  20),  while  in  the  region  of  the  shield 
there  is  no  middle  layer  whatever.  By  the  time  the  primitive  streak  has  ap- 
peared in  the  sheep,  the  formation  of  the  mesoderm  has  extended  under  the  em- 
bryonic shield,  and  the  relations  between  the  germ-layers  then  become  essen- 
tially as  above  described. 

The  cells  of  the  mesoderm  are  at  first  quite  closely  packed,  but  as  the  layer 
grows  they  begin  to  move  apart,  though  remaining  connected  with  one  another 
by  protoplasmic  processes.  The  moving  apart  of  the  cells  is  least  near  the  primi- 
tive streak  and  becomes  more  marked  as  we  go  toward  the  periphery  of  the  layer 
(Fig.  21),  which  represents  a  part  of  the  peripheral  region  of  the  mesoderm  of  a 
blastodermic  vesicle  of  a  rabbit  of  seven  days. 

In  the  details  of  its  expansion  the  mesoderm  varies  greatly  in  different 
mammals.  In  some  forms  it  develops  very  early  and  rapidly  expands  over  the 
entire  blastodermic  vesicle,  which  then  becomes  three-layered.  This  seems  to 
be  the  method  of  its  growth  in  man  and  other  primates.  In  other  cases,  as  in  the 
dog  and  cat,  it  grows  more  slowly,  but  ultimately  encloses  the  entire  entoderm. 
In  the  rabbit,  on  the  contrary,  it  never  expands  more  than  about  three-fifths  of 
the  way  over  the  blastodermic  vesicle,  one  part  of  which,  therefore, — viz.,  that 
opposite  the  embryo, — never  has  any  mesoderm  whatever.  This,  however,  is  to 
be  regarded  as  a  special  modification,  since  we  must  consider  that  primitively 
the  mesoderm  extended  over  the  entire  vesicle. 

The  Primitive  Axis. 

The  next  stage  of  development  is  characterized  by  the  appearance  of  an 
accumulation  of  cells  which  extends  forward  from  the  primitive  knot  in  the  axial 
line.  This  thickening  is  termed  the  primitive  axis.  German  writers  commonly 
designate  it  as  the  " head  process"  (Kopffortsatz).  The  primitive  axis  may  be 
easily  distinguished  in  transverse  sections  from  the  primitive  streak  by  the  fact 
that  in  the  former  the  thickening  occurs  in  the  mesoderm  and  entoderm,  which 
are  closely  united,  and  it  is  separated  from  the  outer  layer;  whereas  in  the  latter 
the  cells  of  the  thickening  are  fused  with  both  the  entoderm  and  the  ectoderm. 

The  primitive  axis  corresponds  to  the  region  in  which  the  body  proper  of 
the  embryo  develops,  and  represents  the  beginning  of  embryonic  development  in 
this  restricted  sense.  It  grows  quite  rapidly  in  length  and  width,  and  as  it  grows 
encroaches  more  and  more  upon  the  territory  of  the  primitive  streak,  which  is 
gradually  obliterated  by  merging  into  the  caudal  end  of  the  developing  embryo, 
so  that  it  can  no  longer  be  distinguished.  The  obliteration  of  the  primitive  streak 
is  gradual,  and  there  is  a  series  of  stages  easily  observed  in  amniota  in  which 
we  find  the  embryonic  development  in  the  region  of  the  primitive  axis  more  or 
5 


66 


THE  EARLY  DEVELOPMENT  OF  MAMMALS. 


less  advanced,  while  part  of  the  primitive  streak  still  presents  to  us,  more  or  less 
clearly,  its  original  condition. 

The  Notochordal  Canal. 

In  regard  to  this  canal  our  knowledge  is  imperfect.  Any  account  of  it 
which  we  can  give  may  need  correction.  It  is  a  very  small  canal  which  runs 
through  the  center  of  the  primitive  axis.  It  ends  blindly  in  front,  but  opens 
through  the  ectoderm  at  its  posterior  end,  at  a  point  corresponding  perhaps 
exactly  to  the  position  of  the  primitive  knot.  The  first  indication  of  the  forma- 
tion of  the  canal  is  an  alteration  in  the  form  of  the  cells  in  the  center  of  the  primi- 
tive axis.  These  cells  elongate  in  directions  at 
right  angles  to  the  axis.  Their  nuclei  become  oval 
and  are  radially  placed.  The  change  begins  poste- 
riorly and  progresses  forward.  The  radial  cells 
move  apart,  so  that  there  arises  a  longitudinal 
canal.  It  may  happen  -that  in  mammals,  as  in 
birds,  the  canal  is  not  actually  open  at  its  posterior 
end.  If  that  should  be  found  to  be  the  case  in  any 
instance,  it  would  not  alter  our  interpretation,  for 
we  should  then  consider  that  the  walls  had  simply 
closed  together.  There  are  many  instances  of  tub- 
ular structures  being  temporarily  solid  in  embryonic 
stages.  Such  a  condition,  for  example,  has  been 
observed  in  the  oesophagus  of  elasmobranchs,  in 
the  large  intestine  of  birds,  and  in  other  cases. 

The  opening  of  the  notochordal  canal  is  termed 
the  blastopore,  and  is  supposed  to  be  identical  with 
the  blastopore  of  the  anamniota. 

After  the  notochordal  canal  is  formed  the 
blastodermic  vesicle  has,  of  course,  two  cavities: 
first,  the  small  cavity  of  the  canal;  second,  the 

large  main  cavity  of  the  vesicle  which  is  surrounded  by  entoderm.  This  larger 
space  is  designated  as  the  yolk-cavity.  After  the  canal  has  acquired  a  not  in- 
considerable length  its  lower  wall  develops  a  series  of  irregular  openings  (Fig. 
22,  nch)  on  its  ventral  side,  by  which  it  comes  into  communication  with  the 
large  underlying  yolk-cavity.  These  openings  grow  until  the  ventral  wall  of 
the  notochordal  canal  is  entirely  lost.  We  then  have  the  two  cavities  com- 
pletely fused  making  a  single  cavity,  bounded  by  a  continuous  layer  of  cells, 
the  majority  of  which  represents  the  lining  of  the  yolk-cavity,  but  the  small 


FIG.  22. — GERMINAL  AREA  OK  A 
GUINEA-PIG  AT  THIRTEEN  DAYS 
AND  TWENTY  HOURS,  SEEN 
FROM  THE  UNDER  (ENTODER- 
MAL)  SIDE. 

a.o,  Areaopaca.  a.p,  Area  pellucida. 
nek,  Notochordal  canal  with 
several  irregular  openings  through 
the  entoderm. 


THE  NOTOCHORD. 


67 


minority  represents  the  cells  of  the  notochordal  canal.  The  continuous  layer 
of  cells  is  known  as  the  permanent  entoderm.  At  about  this  time,  probably 
sometimes  earlier,  sometimes  later,  according  to  the  species,  the  blastopore 
becomes  permanently  closed  and  the  entodermal  cavity  no  longer  has  an  open- 
ing to  the  exterior. 

The  cells  on  the  dorsal  side  of  the  notochordal  canal  have  a  different  destina- 
tion, for  they  become  thickened  to  make  the  anlage  of  the  future  notochord.  It 
is  to  this  fact  that  the  canal  owes  its  name. 

The  Notochord. 

The  notochord  (chorda  dorsalis)  is  a  rod  of  peculiar  tissue  constituting  the 
primitive  axial  skeleton  of  vertebrates.  It  begins  in  the  embryo  immediately 
behind  the  pituitary  body  and  extends  to  the  caudal  extremity.  It  occurs  as  a 
permanent  structure  in  some  of  the  lower  vertebrates  and  as  a  temporary  one  in 
the  embryos  of  amniota.  It  appears  very  early  in  the  course  of  development, 
being  differentiated  from  the  median 
dorsal  wall  of  the  notochordal  canal, 
beginning  at  a  time  when  the  medul- 
lary groove  (compare  page  70)  is  not 
fully  marked  out  posteriorly,  and  is 
nowhere  closed.  The  notochordal 
anlage  can  be  first  detected  as  an 
axial  band  of  cells,  which  at  first  is 
not  well  marked  off  from  the  meso- 
derm of  the  primitive  axis.  The  cells  FIG.  23.— TRANSVERSE  SECTION  OF  A  MOLE  EMBRYO 
of  the  anlage  are  larger  than  those  (HEAPE'S  STAGE  H). 

am,  Amnion.     Mdt  Medullary  groove.      My,  Primitive 
segment.    Co?,  Ccelom.     En,  Entoderm.    nch,  Noto- 


Vta 


chord.  ao,  Aorta, 
Somatic  mesoderm. 
(After  W.  Heape.) 


vt.a,  Vitelline   artery.     Som, 
Spl,  Splanchnic  mesoderm.  — 


of  the  adjacent  entoderm  (Fig.  23, 
nch) .  The  differentiation  of  the  noto- 
chordal cells  begins  usually  at  the 
anterior  end  of  the  canal  and  pro- 
gresses backward.  It  appears  merely 

as  a  specialized  part  of  the  entoderm  of  the  blastodermic  vesicle,  but  has  a  very 
sharp  demarcation. 

The  notochordal  anlage  separates  off  and  the  entoderm  proper  closes  across 
under  it,  so  that  the  notochordal  band  lies  between  the  entoderm  and  the  over- 
lying ectoderm  (floor  of  the  medullary  groove  or  canal).  The  two  primitive 
germ-layers  come  into  actual  contact  in  the  median  line,  along  which,  therefore, 
when  the  notochord  first  separates  from  the  entoderm,  there  is  no  middle  germ- 
layer  present.  The  separation  of  the  anlage  does  not  take  place  at  the  anterior 


68  THE  EARLY  DEVELOPMENT  OF  MAMMALS. 

extremity  of  the  notochord  until  somewhat  later,  so  for  a  considerable  period 
the  cephalic  end  of  the  notochord  remains  fused  witlrthe  entoderm.  The  sepa- 
ration from  the  entoderm  is  effected  in  mammals  by  the  entoderm  proper  shoving 
itself  under  the  notochord  toward  the  median  line.  When  the  cells  from  one  side 
meet,  those  of  the  other  are  united  with  them  and  form  a  continuous  sheet  of 
entoderm  below  the  notochordal  cells. " 

After  its  separation  the  notochord  is  a  narrow  band  of  cells,  which  starts 
anteriorly  from  the  entoderm  (the  future  lining  of  the  alimentary  tract),  running 
backward  to  the  blastopore.  So  long  as  the  blastopore  is  open  the  notochord 
terminates  in  the  epithelium  lining  it.  For  a  certain  period  the  notochord  con- 
tinues to  grow  tailward  by  accretion  of  cells  from  the  walls  of  the  blastoporic 
passage;  and  after  the  canal  is  permanently  obliterated,  the  notochord  may  still 
continue  to  lengthen  by  acquisitions  at  its  caudal  end  of  additional  cells  from  the 
primitive  streak. 

After  it  is  once  formed  as  a  band  of  cells,  the  notochord  passes  through 
various  changes  of  form,  but  ultimately  becomes  a  cylindrical  rod  with  tapering 
extremities.  It  attains  considerable  size  in  the  embryos  of  most  vertebrates, 
but  in  those  of  placental  mammals  it  is  always  small.  It  is  probable  that  in 
mammals  the  notochord,  when  first  separated  from  the  entoderm,  is  a  broad, 
flat  band,  and  that  this  band  subsequently  draws  together,  diminishing  its 
transverse  and  increasing  its  vertical  diameter  until  it  has  acquired  a  rounded 
form.  Finally  its  outline  becomes  circular  in  cross-section.  This  series  of 
changes  begins  near  the  anterior  end  of  the  notochord  and  progresses  both  for- 
ward and  backward. 

In  later  stages  the  mesoderm  again  grows  across  the  median  line  of  the 
embryo,  completely  surrounds  the  notochord,  and  forms  a  special  sheath  about 
it.  Still  later  the  mesoderm  forms  around  the  notochord  groups  of  cells  which 
we  can  identify  as  the  anlages  of  the  vertebrae  and  of  certain  parts  of  the  skull. 

The  Ultimate  Fate  of  the  Notochord. 

As  the  vertebral  column  develops  the  notochord  degenerates.  It  first  ceases 
to  be  continuous  by  breaking  apart  at  points  corresponding  approximately  to 
the  center  of  each  vertebra.  The  fragments  of  the  notochord  contract  and  form 
little  masses  situated  in  cavities  in  the  intervertebral  spaces.  These  cavities 
have  a  distinct  boundary  and  present  characteristic  forms  in  different  mammals. 
The  notochordal  cells  do  not  fill  the  cavity.  The  sheath  of  the  notochord  is  lost ; 
the  walls  of  the  cells  disappear ;  the  tissue  becomes  granular  and  breaks  up  into 
multinucleate,  irregularly  reticulate  masses  which  are  gradually  resorbed  (Fig. 
24).  Tissue  of  this  character  may  be  easily  observed  in  human  embryos  of  the 


THE  ORIGIN  OF  THE  NERVOUS  SYSTEM. 


69 


A.o 


md.F 


third  and  fourth  month.     The  cavity  in  which  these  notochordal  remnants  are 
lodged  is  supposed  not  to  be  identical  with  the  intervertebral  cavity  of  the  adult. 

The  Origin  of  the  Nervous  System. 

The  first  step  in  the  differentiation  of  the  central  nervous  system  is  the 
formation  of  a  thickening  of  the  ectoderm,  which  is  known  as  the  medullary 
plate,  and  which  begins  to  appear  shortly  after  the  formation  of  the  primitive 
streak.  It  extends  over  the  primitive  axis,  the  primitive  knot,  and  the  anterior 
end  of  the  primitive  streak  (Fig. 
25,  Md),  and  also  extends  some 
distance  to  the  right  and  left  of 
the  axial  line.  It  is  rounded  in 
front  and  behind,  where,  how- 
ever, it  gradually  fades  out.  It 
will  be  remembered  that  the 
ectoderm  of  the  embryonic  shield 
has  at  first  a  considerable  thick- 
ness, for  it  consists  of  cuboidal 
or  low  cylindrical  epithelial  cells. 
The  stage  which  follows  next 

pr.s 


Md 


FIG.  24. — DEGENERATING  TISSUE  OF  THE 
NOTOCHORD  FROM  THE  CENTRAL  POR- 
TION OF  THE  INTERVERTEBRAL  Disc 
OF  A  Cow's  EMBRYO. — (After  Leboucq.} 


FIG.  25. — SURFACE  VIEW  OF  THE  EMBRYONIC 
SHIELD  OF  A  DOG  EMBRYO,  WITH  MEDUL- 
LARY PLATE. 

A.o,  Area  opaca.  A.p,  Area  pellucida.  Kn,  Hen- 
sen's  knot.  Md,  Medullary  plate.  md.F, 
Medullary  furrow,  pr.s,  Primitive  streak.  X 
15  diams. 


after  the  appearance  of  the  primitive  axis  is  characterized  by  the  gradual  thin- 
ning out  of  the  ectoderm  over  the  peripheral  portions  of  the  shield,  while  in  the 
neighborhood  of  the  axial  line  the  full  thickness  of  the  outer  germ-layer  is  not 
only  retained,  but  is  actually  increased.  For  a  time  there  is  a  gradual  passage 
between  thicker  and  thinner  parts,  but  as  development  progresses  the  demarca- 
tion rapidly  becomes  sharper.  At  the  same  time  that  the  medullary  plate  is 
being  thus  differentiated,  the  central  portion  becomes  depressed,  making  the 


70 


THE  EARLY  DEVELOPMENT  OE  MAMMALS. 


deep,  narrow  furrow  which  begins  just  in  front  of  the  primitive  knot  and  ex- 
tends nearly  to  the  anterior  edge  of  the  medullary  plate.  This  axial  depression 
is  known  as  the  dorsal  furrow.  Its  appearance  is  shown  in  cross-section  as 
illustrated  by  Fig.  26.  The  furrow  is  narrow  and  deep.  Its  upper  edge  is 
rounded  or  curving.  By  the  formation  of  the  furrow  the  ectoderm  of  the 
medullary  plate  is  brought  into  actual  contact  with  the  anlage  of  the  noto- 
chord  (Fig.  26,  ch),  so  that  the  mesoderm  can  be  no  longer  in  the  median  line, 
and  is  consequently  divided  into  right  and  left  parts,  as  above  mentioned 
in  describing  the  formation  of  the  notochord.  As  the  blastopore  lies  at 
or  near  the  side  of  the  primitive  knot,  it  becomes  included  in  the  medul- 
lary plate.  It  may  remain  open  while 
the  medullary  plate  is  being  transformed 
into  the  nervous  system,  and  in  that 
case  may  establish  a  connection  be- 
tween the  cavity  of  the  central  nervous 
system  and  that  of  the  entoderm.  Such 
a  communication  is  termed  a  neurenteric 
canal.  Fig.  64  represents  a  wax  model 
reconstructed  from  the  sections  of  a 
human  embryo  in  the  stage  of  the 
medullary  plate.  It  shows  clearly  the 
form  of  the  plate,  the  deep  dorsal  groove, 
the  opening  of  the  neurenteric  canal, 
and  the  remnants  of  the  primitive  groove 
behind  the  canal.  As  the  development 
progresses  the  medullary  plate  extends 
further  backward  and  encroaches  upon 
the  territory  of  the  primitive  streak  until 
this  latter  is  obliterated. 

The  Medullary  Groove. — Almost  or  quite  as  soon  as  the  medullary  plate  is 
formed  its  lateral  portions  begin  to  arise  on  each  side,  so  that  the  two  halves  of 
the  plate  together  form  a  broad  open  trough  known  as  the  medullary  groove,  into 
which,  of  course,  the  dorsal  groove  is  merged,  so  that  it  no  longer  can  be  recog- 
nized. While  the  groove  is  being  formed  the  medullary  plate  increases  consid- 
erably in  thickness.  These  nuclei  multiply  rapidly  and  lie  irregularly  scattered 
at  various  heights.  The  ectoderm  alongside  the  medullary  plate  or  groove  thins 
out  still  further.  The  development  is  most  rapid  at  a  point  corresponding  to  the 
posterior  region  of  the  future  head.  The  further  from  this  point  we  go,  the  less 
advanced  do  we  find  the  formation  of  the  groove,  so  that  at  a  certain  stage  there 


FIG.  26. — CROSS-SECTION  OF  A  HUMAN  EMBRYO 
OF  1.54  MM. 

f,  Dorsal  furrow,  ek,  Ectoderm,  ct,  Somatic  meso- 
derm. /,  Beginning  of  the  embryonic  coelom. 
g,  Junction  of  the  extra-embryonic  somatic  and 
splanchnic  mesoderm.  df,  Splanchnic  meso- 
derm. en,  Entoderm.  me,  Mesoderm.  ch. 
Notochord.  —  (After  Count  Spee.) 


THE  STRUCTURE  OF  THE  MEDULLARY  CANAL.  71 

is  a  well-marked  medullary  groove  in  the  cephalic  region,  the  medullary  plate 
behind  that,  and  the  primitive  streak  at  the  hind  end  of  the  embryo.  But  wlien 
the  streak  has  disappeared,  the  medullary  groove  is  found  to  extend  the  entire 
length  of  the  embryo.  Owing  to  this  peculiarity,  it  is  possible  in  a  single  embryo 
to  follow  all  the  principal  stages  of  the  formation  of  the  medullary  groove  by  the 
examination  of  a  series  of  transverse  sections.  Such  a  stage  is  found  in  the  rabbit 
at  nine  days,  or  in  the  chick  at  from  thirty  to  forty  hours  of  normal  incubation 
(Figs.  167,  168,  and  173). 

The  Medullary  Canal. — The  medullary  groove  gradually  deepens,  its  sides 
rising  higher  and  higher  and  arching  more  and  more  toward  one  another  until 
the  edges  meet  and  coalesce,  thus  changing  the  groove  into  a  tube — the  medul- 
lary canal.  The  closure  of  the  groove  occurs  in  the  cervical  region  first,  and 
spreads  from  there  in  both  directions.  As  the  closure  progresses  forward  it 
completes  the  canal  in  the  region  of  the  head.  It  occurs  in  such  a  manner  that 
there  is  a  very  small  opening,  which  is  the  last  point  to  close.  This  opening 
seems  to  be  a  fixed  point,  occupying  always  the  same  relative  position  in  all 
vertebrates.  It  is  called  the  anterior  neuropore.  At  this  time  the  caudal  end  of 
the  medullary  groove  may  be  still  open,  and  it  is  the  last  portion  to  close.  Of 
the  entire  length  of  the  primitive  canal,  about  one-half  is  the  anlage  of  the  brain, 
while  the  other  half  is  the  anlage  of  the  spinal  cord.  In  the  subsequent  develop- 
ment of  the  brain  the  transverse  expansion  of  the  canal  is  most  conspicuous, 
while  in  the  development  of  the  spinal  cord  the  elongation  of  the  canal  predomi- 
nates. The  dilatation  of  the  brain  begins  very  early. 

The  medullary  canal  produces  the  entire  central  nervous  system.  Some  of 
the  cells  from  its  walls  migrate  out  of  the  wall  itself  on  either  side.  These  cells 
produce  the  ganglia. 

The  Structure  of  the  Medullary  Canal. 

When  the  medullary  canal  is  first  formed,  it  tends  to  present  a  rounded  out- 
line in  transverse  section.  But  its  lateral  walls  being  thicker  than  the  wall  on 
the  dorsal  and  ventral  sides  of  the  canal,  the  internal  cavity  appears  somewhat 
flattened  (Fig.  27).  On  its  ventral  side  it  lies  against  the  notochord.  On  its 
dorsal  surface  it  is  in  contact  with  the  overlying  ectoderm,  from  which  it  has, 
however,  completely  separated,  and  it  causes  the  overlying  ectoderm  to  rise  up 
somewhat.  Its  sides  are  in  contact  with  the  mesoderm,  which  is  there  developing 
into  the  primitive  segments,  page  79.  The  nuclei  in  the  wall  of  the  canal  are 
very  numerous,  oval  in  form,  and  usually  with  a  single  nucleolus.  The  nuclei 
are  placed  in  the  radial  lines.  For  some  time  after  the  canal  has  become  closed 
the  nuclei  multiply  very  rapidly  by  indirect  division,  but  all  of  the  mitotic  figures 


THE  STRUCTURE   OF  THE  MEDULLARY  CANAL. 


73 


are  found  close  to  the  inner  surface  of  the  canal,  which  surface,  it  will  be  remem- 
bered, corresponds  to  the  original  outer  surface  of  the  ectoderm. 

The  differentiation  of  the  brain  and  spinal  cord  is  indicated  even  during  the 
stage  of  the  medullary  groove.  The  extreme  anterior  end  of  the  groove  is  found 
to  widen  out  so  as  to  produce  a  pair  of  lateral  expansions.  As  development 
progresses  and  the  canal  closes,  these  expansions  become  more  marked  and  are 
themselves,  of  course,  also  closed  over,  so  that  when  the  canal  is  completed  they 
appear  as  lateral  diverticula  or  evaginations  of  the  tube,  which  are  known  as  the 
primary  optic  -vesicles.  While  the  vesicles  are  developing  the  medullary  tube 
expands  in  diameter  throughout  its  cranial  or  anterior  half  without  any  notice- 
able change  in  the  general  histological  structure  of  its  walls.  Very  soon  the 


Md        Seg 


Cho, 


Am 


FIG.  28. — TRANSVERSE  SECTION  OF  A  RABBIT  EMBRYO  OF   EIGHT  DAYS  AND  Two  HOURS. 

Md,   Medullary  canal.     Seg,  Primitive  segments.     Cho,  Chorion.     Am,  Amnion.     Som,   Somatopleure.      Co:, 

Ccelom.     Spl,  Splanchnopleure.     Ent,  Entoderm.      Ch,  Notochord.     Ao,  Aorta. 


expansion  becomes  unequal,  and  the  inequalities  are  such  that  they  produce 
three  dilatations,  which  are  known  as  the  three  primary  cerebral  "vesicles.  The 
first  vesicle  is  in  the  region  of  the  optic  outgrowth ;  the  second  is  just  behind 
this,  and  the  third  is  as  long  as  the  first  and  second  combined  and  merges 
into  the  spinal  cord.  At  the  time  these  vesicles  become  recognizable  they 
occupy  about  half  the  entire  length  of  the  medullary,  tube.  Between  the  first 
and  second  vesicles  there  is  a  constriction,  and  one  also  between  the  second  and 
the  third.  The  three  vesicles  are  the  anlages  respectively  of  the  fore-brain, 
mid-brain,  and  hind-brain. 

In  the  region  of  the  spinal  cord  the  medullary  tube  soon  becomes  somewhat 


74  THE  EARLY  DEVELOPMENT  OF  MAMMALS, 

flattened  from  side  to  side,  and  therefore  acquires  a  characteristic  oval  configura- 
tion as  seen  in  cross-section  (Fig.  28).  We  can  now  recognize  in  the  cross-sections 
four  regions :  first,  the  two  thick  sides ;  second,  in  the  median  dorsal  line  the  thin 
portion  which  we  call  the  deck-plate,  and  in  the  median  ventral  line  the  thin  por- 
tion which  we  call  the  floor-plate.  Later  on,  each  lateral  portion  becomes  sub- 
divided into  two  longitudinal  bands,  known  as  the  zones  of  His,  and  distin- 
guished from  one  another  as  the  dorsal  and  ventral  zones.  After  this  stage  there 
are  six  longitudinal  zones  in  the  embryonic  cord.  These  are,  first,  the  deck- 
plate;  second  and  third,  the  dorsal  zones  of  His;  fourth  and  fifth,  the  ventral 
zones  of  His ;  and  sixth,  the  floor-plate.  These  six  zones  also  appear  in  the  region 
of  the  brain,  where,  however,  they  undergo  characteristic  modifications.  The 
zones  of  His  dominate  the  entire  morphology  of  the  central  nervous  system. 

The  Early  History  of  the  Mesoderm. 

Concerning  the  precise  origin  and  early  development  of  the  mesoderm  au- 
thorities are  by  no  means  agreed,  and  in  the  interpretations  offered  there  has 
been  more  of  hypothesis  than  of  observation.  The  most  accurate  observations 
have  so  far  been  made  on  the  elasmobranchs,  lizards  and  chick.  In  these  forms 
the  entoderm  (or  segmenting  yolk)  in  the  neighborhood  of  the  primitive  streak 
produces  cells  which  take  their  place  so  as  to  form  a  layer  next  to  the  entoderm. 
This  layer  gradually  becomes  more  and  more  distinct  until  it  can  be  definitely 
recognized  as  a  separate  layer,  the  mesoderm.  It  is  probable  that  a  similar 
process  goes  on  in  amphibia  and  in  mammals,  so  that  it  is  safe  to  say  that  the 
mesoderm  probably  arises  by  this  process,  which  we  call  delamination,  in  all 
vertebrates.  In  its  first  stage  the  mesoderm  has  no  distinct  boundary  against 
the  underlying  entoderm.  It  is  thickest  in  the  neighborhood  of  the  primitive 
streak  and  thins  out  from  that  in  all  directions.  It  very  early  comprises  two 
easily  recognizable  classes  of  cells.  One  of  these  forms  a  more  or  less  distinct 
layer  next  to  the  yolk,  and  so  distributes  itself  as  to  form  a  network  of  cavi- 
ties of  which  these  cells  become  the  boundaries,  thus  developing  the  first  blood- 
vessels. The  cells  which  form  them  constitute  the  angioblast.  A  portion  of  the 
angioblastic  cells  comes  to  lie  in  the  cavities  of  these  primitive  blood-vessels  and 
is  transformed  into  the  first  red  blood-corpuscles  of  the  embryo.  The  second 
class  of  cells  constitutes  the  mesoderm  proper,  and  forms  a  more  continuous 
sheet  of  undifferentiated,  somewhat  closely  compacted  cells,  extending  out  from 
the  primitive  streak  and  lying  between  the  angioblast  and  the  ectoderm. 

The  Expansion  of  the  Mesoderm. — After  the  mesoderm  is  once  formed  as  a 
distinct  layer,  it  seems  to  have  no  longer  any  connection  with  the  entoderm  or 
ectoderm,  except  in  the  axial  line.  Its  further  expansion  is  due  to  the  prolifera- 


THE  EARLY  HISTORY  OF  THE  MESODERM. 


75 


tion  of  its  own  cells.  During  this  early  expansion  the  mesoderm  assumes  in  all 
amniota  a  definite  and  characteristic  series  of  outlines.  It  is  at  first  pear- 
shaped  (Fig.  29,  A),  the  anterior  end  being  pointed.  It  extends  a  short  distance 
only  in  front  of  the  primitive  streak  and  is  widest  a  little  distance  behind  the  area 
pellucida,  Ap.  (For  a  description  of  the  area  pellucida  see  Chapter  V.)  The 
condition  in  the  chick  at  about  the  twentieth  hour  of  incubation  is  indicated  by 
Fig.  29,  B,  drawn  on  the  same  scale  as  A,  and  at  the  close  of  the  first  day  by  Fig. 
29,  C.  In  the  last  stage  figured  it  will  be  noticed  that  the  mesoderm  is  expand- 
ing unequally  in  front,  haying  sent  two  lateral  wings  which  leave  a  median  space 
between  them  without  mesoderm.  These  wings  continue  their  growth,  and 
finally  meet  in  front,  so  that  in  the  anterior  part  of  the  area  pellucida  there  is  a 
small  tract  without  any  mesoderm,  although  it  is  completely  enclosed  by  meso- 


A 


FIG.  29. — THREE  DIAGRAMS  OF  EMBRYONIC  AREAS  OF  CHICKS  TO  SHOW  THE  GROWTH  OF  THE 

MESODERM. 

The  mesoderm  is  indicated  by  vertical  shading,  the  area  opaca  by  horizontal  shading.     A.o,  Area  opaca. 
A.p,  Area  pellucida.     mes,  Mesoderm.     pr,  Primitive  streak. — (After  Duval.) 
% 

derm.  This  tract  is  the  pro-amnion.  The  actual  expanding  edge  of  the  meso- 
derm is  quite  irregular.  The  regularity  shown  in  Fig.  29  is  entirely  diagram- 
matic. 

The  extent  of  the  growth  of  the  mesoderm  over  the  extra-embryonic  region 
of  the  mammalian  blastodermic  vesicle  is  very  variable.  Usually  it  extends 
completely  around  the  vesicle,  but  in  some  cases,  as  in  the  rabbit,  only  part  way 
(compare  page  65,  ante). 

The  Origin  of  the  Ccelom. — The  next  step  in  the  differentiation  of  the  middle 
germ-layer  is  the  appearance  of  two  slit-like  cavities  in  it,  one  on  each  side. 
These  cavities  do  not  extend  across  the  median  line,  for  when  they  appear  there 
is  no  mesoderm  in  the  median  line  of  the  embryo.  The  coelom  is  the  anlage  of 
the  body-cavity,  and  in  part  persists  in  the  adult  as  thepericardial,pleural,and 


76  THE  EARLY  DEVELOPMENT  OF  MAMMALS. 

abdominal  cavities.  Certain  parts  of  its  walls  share  in  the  production  of  mus- 
cles and  of  the  excretory  organs.  The  complete  history  of  the  coelom  is  very 
complex.  As  the  coelomatic  cavities  appear,  the  cells  bounding  them  take  on  a 
distinctly  epithelial  character.  This  limiting  layer  is  termed  the  mesothelium. 

The  earliest  phases  in  the  development  of  the  coelom  have  been  exactly 
followed  only  in  a  very  few  instances.  In  these  it  has  been  found  that  numerous 
fissures  appear  in  the  mesoderm  and  unite  themselves  so  as  to  form  a  network  of 
channels  which  grow,  and  produce  by  their  fusion  the  coelom.  The  fusion  occurs 
so  that  two  cavities  are  developed,  one  on  either  side,  and  parallel  with  the  axis 
of  the  embryo.  As  the  head  of  the  embryo  grows  the  two  cavities  grow  into  its 
cer-vical  end,  following  the  penetration  of  the  mesoderm,  and  unite  so  as  to  form 
below  the  developing  pharynx  a  single  median  cavity,  the  anlage  of  the  future 
pericardial  cavity.  In  the  Sauropsida  and  in  many  mammals  the  pericardial 
coelom  merges  into  two  large  expansions  of  the  body-cavity  which  lie  just  along- 
side of  the  head  of  the  embryo  and  are  known  as  the  amnio-cardiac  vesicles  (Fig. 
167).  (Compare  also  the  account  of  the  splanchnocoele,  page  82.) 

There  are  very  great  variations  in  the  development  of  the  coelom  in  mam- 
mals. In  some  cases  the  coelom  grows  so  as  to  appear  at  an  early  stage  in  the 
body  of  the  embryo  (Fig.  27).  In  other  cases  it  is  developed  in  the  entire  extra- 
embryonic  region  of  the  blastodermic  vesicle  before  it  is  developed  in  the  embryo 
proper.  This  condition  has  been  observed  in  primates,  including  man.  It 
results  in  the  formation  of  a  layer  of  mesoderm  surrounding  the  yolk-sac,  and 
another  layer  underlying  the  extra-embryonic  ectoderm,  with  a  wide  coelomatic 
space  between  the  two  mesodermic  layers.  This  space  we  call  the  extra-embry- 
onic coelom.  These  relations  are  illustrated  in  Fig.  31. 

As  soon  as  the  coelom  has  appeared  the  mesoderm  is  divided  into  two  layers, 
an  outer  and  an  inner.  The  outer  layer  is  in  close  contact  with  the  ectoderm. 
It  is  called  the  somatic  mesoderm.  The  inner  layer  is  in  close  contact  with  the 
entoderm;  it  includes  the  entire  angioblast,  there  being  in  early  stages  no 
blood-vessels  or  blood  in  the  somatic  mesoderm.  The  inner  layer  is  called  the 
splanchnic  mesoderm. 

Somatopleure  and  Splanchnopleure. 

The  somatic  mesoderm,  together  with  the  overlying  ectoderm,  constitute 
the  somatopleure  or  primitive  body- wall.  The  splanchnic  mesoderm,  together 
with  the  underlying  entoderm,  constitute  the  Splanchnopleure.  The  somato- 
pleure and  Splanchnopleure  are,  to  a  large  degree,  the  elementary  anatomical 
parts  out  of  which  the  adult  structure  is  produced.  Although  they  each  com- 
prise cells  belonging  to  two  germ-layers,  they  nevertheless  develop  each  almost 


SOMATOPLEURE  AND  SPLANCHNOPLEURE. 


77 


.-Ch 


Ent 


as  a  unit,  the  cells  of  the  two  germ-layers  entering  into  intimate  co-operation 
with  one  another  in  the  differentiation  of  organs.  In  both  somatopleure  and 
splanchnopleure  it  is  convenient  to  distinguish  two  main  regions ;  namely,  the 
embryonic,  which  enters  into  the  constitution  of  the  embryo  proper,  and  the 
extra-embryonic,  which  enters  into  the  formation  of  the  so-called  appendages 
of  the  embryo,  that  is  to  say,  of  parts  which  exist  during  embryonic  life,  but  are 
lost  at  the  time  of  birth,  and  take  no  share  in  the  permanent  body. 

In  the  primitive  type  of  vertebrate  development  there  are  no  embryonic 
appendages.      This  condition  is  illustrated   by  Fig.  30,  which  is  a  transverse 
section  of  a  young  stage  of  an  axolotl.     This  may  be  readily  compared  with  a 
blastodermic  vesicle  of  a  mammal,  if  we  im- 
agine the  mass  of  yolk  or  entoderm  reduced 
to  a  single  layer  of  cells.     We  can  then  easily 
distinguish  the  ectoderm  and  the  underlying 
somatic  mesoderm,  which  together  completely 
enclose   the    section.     The   splanchnic   meso- 
derm lies  close  against  the  yolk  and  is  separated 
from  the  somatic  by  the  intervening  coelom. 

The  general  homologies  of  this  primitive 
type  of  vertebrate  embryos  with  the  type  which 
we  find  in  the  amniota  may  be  readily  grasped 
by  the  aid  of  the  accompanying  diagrams  (Fig. 
31),  which  are  based  somewhat  on  the  processes 
as  actually  found  in  the  chick.  The  embryonic 
structures  properly  so  called  are  distinguished 
by  shading.  The  yolk-sac  is  large  and  more 
or  less  a  separate  structure  from  the  embryo. 
It  is  surrounded  by  a  layer  of  mesoderm  repre- 
sented by  a  dotted  line.  In  the  direction  of  the 
embryo  the  mesoderm  has  continued  to  form 

part  of  the  wall  of  the  intestinal  canal,  In;  hence  we  may  say  that  the  splanch- 
nopleure forms  the  wall  of  the  primitive  intestinal  canal  and  of  the  yolk-sac. 
The  yolk-sac  represents  a  lower  portion  of  the  splanchnopleure.  It  can 
be  readily  seen  that  we  may  compare  it  with  the  condition  noted  in  the 
newt,  and  have  to  deal  fundamentally  with  a  question  of  relative  proportions. 
The  somatopleure,  Som,  enters  into  the  formation  of  the  embryo  itself,  but  it 
also  extends  beyond.  Its  disposition  becomes  complicated  in  the  amniota  by 
the  formation  of  the  amnion  itself.  We  shall  consider  here  only  what  is  looked 
upon  as  the  primitive  method  of  the  production  of  the  amnion,  and  note  only 


FIG.  30. — TRANSVERSE  SECTION  OF  AN 
EARLY  STAGE  OF  AN  AXOLOTL. 

EC,  Ectoderm.  mes,  Mesoderm.  Md, 
Medullary  groove.  Ch,  Notochord. 
Ent,  Entoderm.  Yk,  Yolk.  Ach, 
Archenteron  or  primitive  entodermal 
cavity. — (After  Bellonci.~] 


78 


THE  EARLY  DEVELOPMENT  OF  MAMMALS. 


that  the  exact  steps  of  the  process  are  considerably  modified  in  many  mammals, 
in  connection  with  the  early  modifications  which  the  ovum  undergoes  in  order 
to  secure  its  attachment  to  the  walls  of  the  uterus  (see  the  section  on  the 
trophoblast).  The  somatopleure  forms  two  folds,  one  on  each  side  of  the 
embryo.  These  folds  arch  up  over  the  back  of  the  embryo.  The  inner  leaf  or 
part  of  each  fold  is  the  anlage  of  the  amnion,  Am.  It  consists  of  a  layer  of  ecto- 
derm next  to  the  embryo,  and  a  layer  of  mesoderm,  represented  by  the  dotted 
line,  turned  away  from  the  embryo.  The  remaining  portion  of  the  extra- 
embryonic  somatopleure,  Cho,  extends  around  both  the  amnion  and  yolk-sac, 
forming  a  membrane  called  the  chorion,  which  likewise  consists,  of  course,  of 
ectoderm,  which,  however,  faces  away  from  the  embryo,  and  of  mesoderm  (dotted 


Sp] 


Som 


Spl 

Som 


Cho. 


FIG.  31. — GENERALIZED  DIAGRAM  OF  AN  AMNIOTE  VERTEBRATE  EMBRYO. 

The  first  figure  shows  the  condition  before,  the  second  after,  the  separation  of  the  amnion  from  the  chorion. 
Am,  Amnion.  Cho,  Chorion.  Cce,  Coelom.  In,  Intestinal  canal.  Som,  Somatopleure.  Spl,  Splanch- 
nopleure.  Yolk,  Yolk-sac. 

line),  which  is  turned  toward  the  embryo.  As  regards  the  embryo,  therefore, 
the  position  of  the  two  germ-layers  in  the  amnion  is  reversed  in  the  chorion. 
The  two  folds  continue  to  grow  until  they  meet  above  the  back  of  the  embryo 
and  unite.  The  amnion  (Fig.  31,  Am)  has  thus  become  the  closed  membrane 
surrounding  the  embryo,  and  the  chorion,  Cho,  has  become  a  closed  membrane 
surrounding  the  amnion,  the  embryo,  and  the  yolk-sac. 

By  the  processes  indicated  we  have  produced  an  embryo  with  its  three 
primary  appendages — the  chorion,  amnion,  and  yolk-sac.  To  these  there  is  to 
be  added  a  fourth  appendage,  the  allantois,  which  also  begins  its  development 
very  early,  and  arises  as  a  hollow  outgrowth  from  the  under  side  of  the  caudal  end 


THE  EMBRYONIC  CCELOM.  79 

of  the  embryo  and  expands  into  the  extra-embryonic  coelom  or  space  between  the 
yolk-sac  and  the  chorion. 

The  Embryonic  Coelom. 

In  the  body  of  the  embryo  proper  the  coelom  acquires  a  very  complicated 
disposition.  It  forms,  first,  a  series  of  small  cavities  alongside  of  the  medullary 
tube.  The  walls  of  these  cavities  are  termed  the  primitive  segments.  It  forms 
two  large  main  cavities,  which  partially  unite  in  later  stages  on  the  ventral  side 
of  the  embryo,  the  primitive  segments  lying  more  on  the  dorsal  side.  These  two 
large  ccelom  spaces  constitute  the  splanchnocele,  a  term  which  has  reference  to  the 
fact  that  this  space  surrounds  the  splanchnic  viscera.  Finally,  it  forms  a  series 
of  so-called  head-cavities,  of  which  there  are  probably  always  three  on  each  side 
of  the  head.  The  walls  of  these  head-cavities  in  part  produce  the  muscles  of  the 
eye.  We  must  now  consider  the  development  of  these  divisions  of  the  ccelom  in 
the  order  indicated. 

The  Primitive  Segments. — A  segment  consists  of  a  pair  of  cavities  symmetri- 
cally placed  and  bounded  by  mesothelium.  The  cavities  are  portions  of  the  em- 
bryonic ccelom.  For  convenience  of  description  the  term  segment  is  usually 
applied  to  one  of  the  pair  of  structures  which  constitute  a  whole  segment.  The 
primitive  segments  appear  very  early;  the  first  pair  can  be  recognized  in  the 
chick  after  twenty  to  twenty-two  hours'  incubation;  in  the  rabbit,  at  the  begin- 
ning of  the  eighth  day.  In  both  cases  the  medullary  groove  is  still  nowhere 
closed.  In  amniote  embryos,  just  before  the  first  segment  appears,  the  meso- 
derm  on  either  side  of  the  axial  line  is  considerably  thicker  than  further  away 
from  it.  We  can,  therefore,  distinguish  two  zones — namely,  the  thicker  seg- 
mental  zone  near  the  axis,  and  the  thinner,  but  much  wider  lateral  or  parietal 
zone.  The  first  step  in  the  formation  of  the  first  segment  is  a  loosening  of  the 
cells  in  the  segmental  zone,  along  a  narrow  transverse  line.  In  the  chick  this 
occurs  about  0.14  mm.  in  front  of  the  primitive  streak,  at  a  time  when  only  a 
portion  of  the  medullary  groove  is  formed.  Very  soon  there  appears,  close  by, 
a  second  similar  transverse  loosening  of  the  cells.  The  mesoderm  of  the  seg- 
mental zone  is  thus  cleft  twice,  the  mesodermic  cells  between  the  two  clefts 
constituting  the  first  segment,  which  is  somewhat  cuboidal  in  form.  The  first 
segment  appears  in  what  later  becomes  the  occipital  region.  Two  or,  according 
to  some  authorities,  three  segments  are  formed  on  the  cephalic  side  of  the  first 
segment;  and,  meanwhile,  the  number  of  segments  is  also  increasing  on  the 
caudal  side  of  the  first,  but  much  more  rapidly.  The  primitive  segments, 
owing  to  their  form  and  their  proximity  to  the  anlage  of  the  central  nervous 
system,  were  taken  by  early  embryologists  to  be  the  beginnings  of  the  vertebrae, 


80  THE   EARLY  DEVELOPMENT  OF  MAMMALS. 

and  were,  therefore,  called  the  proto-vertebrce.  This  name  is  still  used,  although 
the  idea,  upon  which  it  was  based,  is  known  to  be  erroneous  because  the  primi- 
tive segments  form  much  more  than  the  vertebrae. 

The  association  in  time  of  the  development  of  the  medullary  groove  and 
primitive  segments  is  important.  By  the  formation  of  the  groove  the  space 
between  the  ectoderm  and  entoderm  alongside  the  groove  is  increased,  and  it  is 
this  space  which  gives  the  mesoderm  the  opportunity  to  grow  in  thickness  so  as 
to  form  the  segmental  zone  next  to  the  medullary  groove. 

In  the  amniota  when  the  primitive  segments  are  first  formed  they  contain 
no  actual  cavity,  but  we  must  consider  that  one  is  morphologically  present,  since 
we  can  easily  observe  the  line  of  contact  between  the  opposite  walls  of  the  seg- 
ments. As  observed  in  transverse  sections  the  segments  when  first  developed 
are  triangular  in  outline.  The  base  of  the  triangle  extends  along  the  side  of  the 


OOltl 


ipr 

FIG.  32. — TRANSVERSE  SECTION  FROM  A  CHICK  EMBRYO  WITH  ABOUT  EIGHTEEN  SEGMENTS. 
Only  the  mesoderm  of  one  side  has  been  drawn.     The  section  passes  through  a  recently  formed  segment.     My, 
Secondary  segment.     C,  Core   of  the  segment.      W.d,     Wolffian  duct.      Nt  Nephrotome.      Cce,   Ccelom. 
Som,   Somatic  mesoderm.     Spl,   Splanchnic  mesoderm.     X  227  diarns. 

medullary  canal;  the  apex  of  the  triangle  lies  next  to  the  splanchnocele,  and  at 
the  point  of  the  triangle  the  somatic  and  splanchnic  mesoderm  separate  widely 
from  one  another.  Very  soon  the  apex  of  the  triangle  forms  a  narrower  piece 
(Fig.  32,  N),  which  is  known  commonly  as  the  nephrotome  or  intermediate  cell 
mass.  While  the  nephrotome  is  being  marked  off  the  proximal  portion  of  the 
segment  enlarges,  the  cells  assume  a  more  distinctly  epithelial  character  (Fig. 
32,  My},  enclosing  a  considerable  space,  which,  however,  is  completely  filled  by 
a  mass  of  cells,  C,  which  arise  by  a  proliferation  of  the  cells  from  the  lower  side  of 
the  segment.  The  line  around  this  mass  of  cells  marking  it  off  from  the  other 
wall  of  the  segment  indicates  the  morphological  cavity.  In  the  sheep  and  the 
chick  it  has  been  observed  that  the  cavities  of  the  first  four  segments  can  be 
traced  through  the  nephrotome  to  the  eplanchnocele.  This  represents  a  primi- 
tive condition,  one  which  we  find  in  all  the  segments  of  some  of  the  lower  verte- 


THE  EMBRYONIC  C(ELOM. 


81 


brates.  Did  we  know  the  development  of  the  amniota  only,  we  should  not  have 
been  able  to  identify  the  cavity  of  the  segment  as  morphologically  a  portion  of 
the  coelom.  The  development  in  fishes  shows  conclusively  that  it  must  be  so 
regarded. 

The  Separation  of  the  Nephrotome. — The  nephrotome  early  loses  its  connec- 
tion on  the  one  side  with  the  enlarged  central  portion  of  the  segment,  and  on  the 
other  with  the  mesodermic  walls  of  the  splanchnocele,  so  that  each  nephrotome 
forms  a  little  mass  of  cells  isolated  from,  but  in  definite  topographical  relation  to, 
the  other  parts  of  the  mesoderm.  It  may  be  noted  that  during  these  early 
stages  one  can  always  find  the  anlage  of  the  Wolffian  duct  on  the  ectodermal  side, 
and  on  the  entodermal  side  the  anlage  of  a  blood-vessel.  Very  soon  the  nephro- 


Am. 


Som.  Ao.  Nek. 

FIG.  33. — SECTION  OF  A  VERY  YOUNG  CAT  EMBRYO.     (Transverse  Series  413,  section  181.) 
Am,  Amnion.      Ao,   Aorta.      Md,   Medullary  tube  (spinal  cord).      My,  Outer,  My' ,  inner  wall  of  primitive 
segment.       Nch,    Notochord.      ATephr,    Nephrotome     (segmental    vesicle).       Som,    Somatopleure.       Spl, 
Splanchnopleure.      Ve,  Blood-vessel.      W.D,  Wolffian  duct.     X  5°  diams. 

tome  assumes  a  rounded  form,  and  a  cavity  appears  in  its  interior;  it  is  then 
often  called  a  segmental  -vesicle  (Fig.  33,  Nephr).  The  exact  details  of  the  process 
by  which  the  nephrotome  is  separated  from  the  other  parts  of  the  middle  germ- 
layer  have  not  yet  been  carefully  studied.  Each  nephrotome  is  the  anlage  of 
one  of  the  excretory  tubules  of  the  Wolffian  body. 

'  The  portion  of  the  primitive  segment  which  is  isolated  by  the  formation  of 
the  nephrotome  lies,  of  course,  next  to  the  medullary  canal.  The  term  primitive 
segment  (as  also  proto- vertebra)  is  often  applied  to  this  structure  as  well  as  to 
the  original  primitive  segment  before  the  separation  of  the  nephrotome,  but  it 
would  be  better  to  refer  to  it  as  the  secondary  segment.*  The  secondary  seg- 

*  This  is  a  new  term,  here  proposed  for  the  first  time. 


82  THE  EARLY  DEVELOPMENT  OF  MAMMALS. 

ment,  when  first  formed,  appears  more  or  less  nearly  square  in  surface  views,  and 
triangular  in  cross-sections.  As  the  medullary  canal  grows,  so  does  the  secondary 
segment,  and  it  becomes,  therefore,  somewhat  elongated  in  its  dorso-ventral 
diameter.  After  this  change  in  its  shape  we  can  distinguish  in  transverse  sec- 
tions of  an  embryo  (Fig.  28)  the  outer  wall,  which  lies  under  the  ectoderm,  and 
an  inner  wall,  which  lies  toward  the  medullary  canal  and  notochord.  In  the 
further  history  of  the  secondary  segment  we  can  distinguish  the  following  steps : 
First,  the  production  of  the  myotome,  with  the  accompanying  transformation  of 
a  portion  of  the  cells  of  the  inner  wall  of  the  segment  into  the  mesenchyma ; 
next,  the  production  of  the  true  muscle  plate;  third,  the  breaking-up  of  the  outer 
wall  of  the  myotome.  These  portions  are  sufficiently  described  in  the  practical 
part,  Chapter  V. 

The  Splanchnocele.— The  splanchnocele  makes  its  first  appearance  in  the 
parietal  zone  of  the  mesoderm  in  the  manner  above  described.  It  rapidly  in- 
creases in  size,  so  that  a  considerable  space  separates  the  somatic  from  the 
splanchnic  mesoderm,  as  shown  in  Fig.  165  and  Fig.  162.  When  it  first  appears, 
it  is  a  narrow  fissure.  It  rapidly  widens,  extends  toward  the  axis  until  it  almost 
reaches  the  primitive  segments,  and  also  spreads  out  laterally  into  the  so-called 
extra-embryonic  region.  As  above  stated,  the  rate  and  extent  of  its  extra- 
embryonic  development  vary  greatly  in  different  mammals.  It  develops  earlier 
and  acquires  a  greater  distention  at  first  in  the  future  cervical  region,  where  it 
produces  the  amnio-cardiac  vesicles,  in  the  median  portion  of  whose  united  cavi- 
ties the  heart  is  lodged.  The  splanchnocele  of  the  body  proper  appears  after  the 
primitive  segments,  and  its  expansion  takes  place  at  first  only  in  the  part  of  the 
mesoderm  next  to  the  primitive  segments.  Everywhere  as  the  splanchnocele 
develops  the  mesodermal  cells  about  it  assume  gradually  more  and  more  dis- 
tinctly an  epithelial  character,  so  that  it  soon  becomes  proper  to  speak  of  the 
mesothelium  or  boundary  epithelial  wall  of  the  coelom. 

The  splanchnocele  is  also  designated  by  several  other  names,  and  is  some- 
times called  simply  the  body-cavity  or  somatic  cavity.  Others  term  it  the  ventral 
ccelom.  By  English  embryologists  it  is  usually  called  the  pleuro- peritoneal  space. 
Its  future  subdivisions  become  early  indicated  by  a  transverse  ridge  of  tissue 
which  is  known  as  the  septum  transversum.  This  septum  is  situated  at  the  poste- 
rior end  of  the  heart,  and  is  developed  to  allow  the  great  veins  to  have  access  to 
the  heart  itself.  It  is  the  anlage  of  the  future  diaphragm.  It  separates  the 
coelom  around  the  heart  from  that  of  the  abdomen.  It  is  a  product  of  the  splanch- 
nopleure,  so  that  it  rises  up  on  the  ventral  side  of  the  coelom.  We  have,  as 
soon  as  this  septum  is  present,  the  pericardial  cavity  on  its  cephalic  side,  the 
abdominal  cavity  on  its  caudal,  and  a  small  pleural  cavity  on  its  dorsal  side. 


THE  MESENCHYMA.  83 

The  Coelom  of  the  Head. — No  adequate  investigation  of  the  early  stages  of 
the  mesoderm  in  the  head  of  amniota  has  yet  been  made.  We  know,  however, 
that  in  the  lower  vertebrates  there  appear  at  least  three  distinct  cavities  re- 
sembling portions  of  the  true  coelom  and  bounded  by  epithelial  cells,  similar  to 
the  mesothelium  in  character.  These  cavities  are  generally  regarded  as  portions 
of  the  true  coelom,  and  by  many  writers  have  been  interpreted  as  true  primitive 
segments.  But  this  interpretation  is  not  yet  beyond  doubt.  The  largest  of 
these  cavities  is  called  the  mandibular,  because  it  has  a  prolongation  which  ex- 
tends into  the  mandible  of  the  young  embryo.  In  front  of  it  is  the  first  or  pre- 
mandibular  cavity,  which  is  much  smaller,  and  behind  it  is  the  third  or  hyoid 
cavity,  which  is  intermediate  in  size  between  the  first  and  second.  The  head- 
cavities  are  best  known  in  the  elasmobranchs.  They  have  also  been  found 
clearly  developed  in  reptiles  and  certain  birds.  In  mammals  no  actual  cavities 
have  been  recorded.  There  are  found  the  anlages  *  of  the  muscles  of  the  eye, 
and  these  are,  by  hypothesis,  homologous  with  the  cells  of  the  walls  of  the  head- 
cavities  in  lower  vertebrates,  which  cells  produce  the  muscles  of  the  eye. 

The  Mesenchyma. 

By  the  term  mesenchyma  we  designate  the  whole  of  the  mesoderm  of  the 
embryo,  except  the  mesothelial  lining  of  the  ccelom.  When  fully  differentiated 
histologically,  it  consists  of  more  or  less  widely  separated  cells,  connected  with 
one  another  by  intervening  threads  of  protoplasm,  which  form  a  network  be- 
tween the  cells.  The  remaining  space  is  filled  by  a  homogeneous  structureless 
matrix  or  basal  substance.  It  gives  rise  to  a  large  number  of  adult  tissues,  as 
shown  in  the  table  on  page  35. 

In  the  early  development,  or  histogenesis,  of  the  mesoderm  we  can  distin- 
guish four  stages:  First,  that  of  distinct  cells;  second,  the  formation  of  the 
cellular  network;  third,  the  formation  of  the  mesothelium;  and,  fourth,  the 
differentiation  of  the  mesenchyma.  The  first  stage  is  known  chiefly  through 
observations  on  the  early  stages  of  elasmobranchs,  reptiles,  and  birds.  In  these 
types  the  first  cells,  which  are  delaminated  from  the  entoderm  to  form  the  anlage 
of  the  mesoderm,  are  of  quite  large  size  and  lie  between  the  entoderm,  or  yolk, 
and  ectoderm,  and  are  without  connection  with  one  another.  The  number  of 
mesodermic  cells  increases  both  by  the  multiplication  of  the  cells  already  de- 
laminated,  and  by  the  addition  of  others  from  the  entoderm.  Whether  this 
stage  occurs  in  mammals,  or  not,  we  do  not  know  at  present.  In  the  second 

*  The  anlages  may  be  seen  in  a  pig  embryo  of  10  mm.  between  the  jugular  vein  and  the  internal  carotid 
artery  as  a  group  of  embryonic  cells  quite  distinct  from  the  surrounding  mesenchyma. 


84  THE  EARLY  DEVELOPMENT  OF  MAMMALS. 

stage  the  primitive  cells  are  found  to  have  acquired  connection  with  one  another, 
the  protoplasm  of  one  cell  uniting  by  a  process,  or  prolongation,  with  the  proto- 
plasm of  another  cell,  and  so  on  until  the  whole  tissue  becomes  a  network.  When 
the  primitive  streak  has  been  formed  in  the  mammalian  blastodermic  vesicle, 
we  find  the  mesoderm  in  this  condition.  The  third  stage  is  brought  about  by 
the  development  of  the  coelom  as  above  described,  and  it  seems  probable  that  all 
the  cells  of  the  mesoderm  are  transformed  into  mesothelium.  But  this  prob- 
ability is  not  at  present  wholly  beyond  question.  It  is  certain  that  nearly,  if  not 
quite,  all  the  mesodermic  cells  become  mesothelium.  To  produce  the  fourth 
stage,  single  cells  leave  the  mesothelium  or  migrate  out  of  it  on  the  side  away 
from  the  coelom.  These  cells  are  found  to  be  connected  both  with  one  another 
and  with  the  mesothelial  cells  by  protoplasmatic  processes,  but  they  do  not  lie 
close  together,  as  in  the  epithelium,  so  that  there  is  a  considerable,  though  vari- 
able, amount  of  intercellular  space.  By  the  migration  of  the  cells  and  their 
multiplication,  the  mesenchyma  is  produced.  It  fills  up  all  the  room  between 
the  mesothelium  and  the  two  primary  germ-layers  so  far  as  it  is  not  occupied  by 
the  developing  blood-vessels. 

Apparently  the  entire  mesothelium  may  participate  in  the  production  of  the 
mesenchymal  cells.  Its  different  regions,  however,  do  not  so  participate  all  to 
an  equal  degree,  or  at  the  same  time.  The  throwing  off  of  mesenchymal  cells 
may  be  observed  in  certain  parts  of  the  embryo  in  somewhat  advanced  stages  of 
development,  and  it  seems  not  impossible  that  the  process  may  be  found  to 
occur  even  in  adult  life. 

The  mesoderm,  by  the  formation  of  mesenchyma,  becomes  very  early  unlike 
the  other  germ-layers.  Both  ectoderm  and  entoderm  are  epithelial  membranes. 
The  mesoderm  is  partly  epithelial,  partly  mesenchymal,  and  from  the  mesen- 
chyma arise  special  kinds  of  tissue  which  are  characteristic  of  the  middle  germ- 
layer,  and  never  are  produced  from  either  the  outer  or  inner  germ-layers. 

The  Germ=cells. 

Concerning  the  primitive  origin  of  the  germ-cells  in  vertebrates  our  knowl- 
edge is  scanty.  The  most  accurate  information  we  have  refers  to  their  develop- 
ment in  the  dog-fish.  In  this  species  the  germ-cells  are  delaminated  from  the 
entoderm  together  with  other  cells  of  the  mesoderm,  and  cannot,  with  our  present 
knowledge,  be  distinguished  from  other  mesodermic  cells.  They  soon,  however, 
became  recognizable,  because  while  the  majority  of  the  mesodermic  cells  are 
passing  into  the  second  stage  (compare  the  section  on  Mesenchyma,  page  83) 
these  germ-cells  change  but  little,  if  at  all,  so  that  they  can  be  recognized  as 
something  distinct  from  the  neighboring  cells.  For  a  short  time  they  are  found 


THE   YOLK-SAC. 


85 


So 


Coe 


gathered  into  two  compact  groups,  symmetrically  placed  in  the  extra-embryonic 
region,  but  not  far  from  the  embryo.  The  cells  then  break  apart  from  one 
another  and  gradually  become  separated,  and  migrate  by  unknown  means,  first 
over  the  wall  of  the  intestine,  which  has  meanwhile  been  differentiated,  then 
over  the  surface  of  the  mesentery  into  the  anlage  of  the  genital  gland.  During 
their  entire  migration  they  are  lodged  in  the  mesothelium,  and  when  they  have 
reached  their  final  destination  they  are  still  in  the  mesothelium  of  the  genital 
anlage,  where  they  remain  until  finally  differentiated  in  the  adult.  The  epithe- 
lium, with  the  germ-cells  in  their  definite  position  in  it,  is  called  the  germinal 
epithelium  (compare  page  39).  The  germinal 
epithelium  has  been  observed  in  all  vertebrates, 
but  the  origin  of  the  germ-cells  in  amniota  is 
entirely  unknown.  The  hypothesis  may  be  ac- 
cepted, that  they  arise  in  a  manner  essentially 
similar  to  that  known  in  the  dog-fish.  For  some 
of  the  theories  based  on  the  known  develop- 
ment of  the  germ-cells,  see  page  40. 

The  Yolk=sac. 

General  Morphology. — The  yolk-sac  is  the 
container  of  the  nutritive  yolk  destined  to  be 
assimilated  by  the  embryo.  The  principal 
factor  in  its  morphological  constitution  is  the 
entoderm,  which,  after  the  differentiation  of  the 
definitive  germ-layers,  contains  nearly  all  of  the 
yolk  material.  In  the  primitive  vertebrates, 
as  exemplified  by  the  marsipobranchs,  ganoids, 
dipnoi,  and  amphibia,  we  find  this  yolk  material 
lodged  in  the  walls  of  the  primitive  digestive 
tract.  It  is  situated  chiefly  on  the  ventral  side  of  this  tract,  and  extends  from  the 
point  where  the  heart  is  formed  toward  the  tail  of  the  embryo  to  the  point  where 
the  allantois  is  formed.  In  other  words,  it  is  situated  in  a  region  corresponding 
to  the  territory  of  the  future  abdominal  cavity.  In  the  primitive  types  just  re- 
ferred to,  the  yolk-bearing  entoderm  becomes  divided  into  distinct  cells  which 
form  a  large  mass.  The  condition  may  be  understood  from  Fig.  30,  which  repre- 
sents a  transverse  section  of  the  early  stage  of  an  axolotl  embryo.  The  cavity 
of  the  entodermal  canal  (digestive  tract)  is  small.  It  is  bounded  on  its  dorsal 
side  by  a  single  layer  of  cells  distinctly  epithelial  in  their  development,  and 
on  the  ventral  side  by  a  great  mass  of  rounded  cells  heavily  laden  with  yolk 


FIG.  34. — DIAGRAMMATIC  SECTION  OF 
THE  YOLK  OF  A  HEN'S  EGG  AT 
AN  EARLY  STAGE  TO  SHOW  THE 
RELATION  OF  THE  PRIMITIVE  ENTO- 
DERMAL CAVITY,  Ach. 

Cm,  Coelom.  In,  Intestinal  cavity.  Som, 
Somatopleure.  Spl,  Splanchnopleure. 


86 


THE  EARLY  DEVELOPMENT  OF  MAMMALS. 


granules,  and  containing  conspicuously  large  nuclei.  These  large  nuclei  differ 
by  their  size  and  minute  structure  very  much  from  the  other  nuclei  in  the  em- 
bryo. The  corresponding  nuclei  in  higher  animals  are  sometimes  called  para- 
blast  nuclei.  Outside  of  the  entoderm  comes  the  second  portion  of  the  yolk-sac, 
the  splanchnic  leaf  of  the  mesoderm.  If  we  imagine  the  amount  of  yolk  to  be 
gradually  increased,  so  that  it  would  appear  more  distinct  from  the  embryo 
proper,  we  should  then  apply  to  it  the  term  extra-embryonic.  The  yolk-sac  of 
the  higher  forms  differs  from  that  of  the  lower  forms  only  by  its  size,  as  is  illus- 
trated by  Fig.  34,  which  represents  a  diagrammatic  transverse  section  of  an 
early  stage  of  the  chick,  before  the  formation  of  the  amnion  has  begun.  The 
essential  relations  may  be  seen  by  comparing  Figs.  30,  34,  and  31.  As  shown  in 
the  section,  Fig.  34,  the  yolk-sac,  if  we  may  so  call  it,  is  completely  enclosed 
by  the  somatopleure  of  the  embryo,  and  in  the  amniote  embryo  the  condition  is 


Ent 


FIG.  35. — WALL  OF  THE  YOLK-SAC  IN  THE  REGION  OF  THE  AREA  OPACA  OF  A  CHICK  OF  THE  SECOND  DAY. 
Jlfes,  Mesoderm.      V,  V,  Blood-vessels,  containing  a  few  young  blood-cells.      Ent,  Entoderm.      c,  Four  distinctly 

shown  entodermal  cells. 


the  same.  The  yolk-sac  is  surrounded  by  the  somatopleure,  which,  however,  in 
the  amniota  we  call  extra-embryonic.  The  extra-embryonic  somatopleure 
around  the  yolk-sac  is  called  in  birds  the  membrana  serosa,  and  in  mammals  the 
chorion. 

In  amniota  we.can  distinguish  in  the  entoderm  of  the  embryo,  or  yolk-sac, 
three  distinct  regions.  The  first  of  these  includes  the  whole  of  the  entoderm  of 
the  embryo  and  a  certain  territory  around  it.  In  this  region,  after  the  earliest 
stages  are  passed,  the  entoderm  is  found  to  be  a  very  thin  layer  and  to  contain 
very  few  yolk-granules,  and  such  few  as  it  contains  are  small.  This  portion  of 
the  entoderm,  therefore,  seems  translucent,  an  appearance  which  can  easily  be 
noted  with  the  naked  eye,  and  which  has  led  to  the  name  area  pellucida,  which 
has  long  been  applied  to  this  region.  The  region  all  around  the  area  pellucida 


THE  YOLK-SAC.  87 

appears  in  the  fresh  specimen  darker,  and  this  is  called  the  area  opaca,  the  second 
region.  The  entoderm  in  this  part  consists  of  columnar  cells  (Fig.  35,0,  and  Fig. 
36).  In  the  chick  the  cells  are  high  cylinder  cells  of  somewhat  irregular  shape, 
containing  a  loose  network  of  granular  protoplasm.  The  lower  ends  of  the  cells 
are  rounded  and  projecting,  and  have  a  well-marked  border  of  dense  protoplasm. 
The  nuclei  are  variable  in  size,  but  for  the  most  part  large,  often  three  or  four 
times  greater  in  diameter  than  the  neighboring  mesodermic  nuclei.  They  usu- 
ally have  one,  sometimes  two,  conspicuous  nucleoli.  The  nuclei  always  lie 
at  the  upper  or  basal  ends  of  the  cells,  chiefly  near  one  side  of  the  cell.  The  cells 
contain  yolk-grains  which  appear  to  be  undergoing  resorption.  Toward  the  area 
pellucida  the  cells  are  smaller,  the  network  of  protoplasm  closer,  and  the  yolk- 
grains  are  either  absent  altogether  or,  if  present,  small  in  size  and  few  in  number. 
The  transition  to  the  thin  entoderm  of  the  area  pellucida  is  quite  abrupt.  In 


v 


FIG.  36. — WALL  OF  THE  YOLK-SAC  IN  THE  REGION  OF  THE  AREA  OPACA  OF  A  RABBIT  EMBRYO  OF 

THIRTEEN  DAYS. 
Fj  Blood-vessels  containing  young  red  blood-cells,  bl.     mes,  Mesoderm. 

the  opposite  direction  the  area  opaca  passes  gradually,  by  changing  its  structure, 
into  the  general  mass  of  the  yolk,  or  area  mtellina,  the  third  of  the  regions  of  the 
yolk-sac,  so  called  because  it  contains  the  bulk  of  the  yolk  material.  The  transi- 
tion of  the  area  opaca  into  the  area  vitellina  is  marked  by  a  considerable  accumu- 
lation of  cells  which  are  arising  from  the  yolk.  This  accumulation  of  cells 
is  called  the  germinal  wall.  It  is  the  connecting-link  between  the  epithelium 
on  the  dorsal  side  of  the  entodermal  cavity  and  the  yolk  or  area  vitellina, 
which  forms  the  ventral  boundary  of  the  cavity.  If  we  follow  successively  the 
stages,  we  find  that  the  area  pellucida  grows  at  the  expense  of  the  area  opaca, 
and  the  area  opaca  at  the  expense  of  the  area  vitellina.  These  facts  are  to  be 
interpreted  as  phases  in  the  process  of  the  assimilation  of  the  nutritive  yolk. 
The  thin  cells  of  the  area  pellucida  are  those  in  which  the  absorption  of  the  yolk 
has  been  completed.  The  larger  cells  of  the  opaca  are  those  in  which  the  assimil- 


88  THE  EARLY  DEVELOPMENT  OF  MAMMALS. 

ation  is  going  on,  and  it  can  be  easily  seen  that  it  is  most  advanced  in  those  cells 
which  are  nearest  the  embryo  and  least  advanced  in  those  cells  which  are  nearest 
to  the  germinal  wall.  In  mammals  the  area  pellucida  is  well  marked  and  re- 
sembles that  of  birds.  The  area  opaca  has  well-defined  cylinder  cells  (Fig.  36) 
which  have  rounded  ends,  but  are  much  smaller  than  in  birds  and  contain  very 
little  yolk  material.  Cells  of  this  character  extend  over  also  what  we  should  call 
the  area  vitellina,  which  does  not  present  the  special  features  which  it  has  in 
birds,  for  the  reason  that  the  yolk  in  mammals  is  so  small  in  amount.  Later  on 
the  cells  pass  through  degenerative  changes,  which  need  to  be  more  exactly 
studied.  In  man  the  degenerative  change  in  the  cells  of  the  yolk-sac  takes  place 
very  early.  The  mesoderm  of  the  yolk-sac  is  at  first  a  thin  layer.  Very  early 
there  appears  an  angioblast,  or  the  anlage  of  the  first  blood-vessels  and  blood. 
In  all  cases  in  which  the  process  has  been  accurately  followed  the  angioblast 


FIG.  37. -SECTION  OF  THE  YOLK-SAC  OF  A  YOUNG  FlG.  38._HuMAN  EMBRYO,  2.15  MM.  LONG.— 

HUMAN  EMBRYO.  (AJter  W.  His.} 

Ent,  Entoderm.     mts,  Mesoderm.      v,  Blood- 
vessels.— (After  Keibel.} 

makes  its  first  appearance  in  the  region  of  the  area  opaca,  where  it  forms  a  net- 
work of  primitive  blood-vessels  close  against  the  surface  of  the  yolk.  The  region 
occupied  by  these  blood-vessels  is  called  the  area  -vasculosa.  Its  boundary  in 
the  direction  away  from  the  embryo  is  everywhere  well  defined.  Gradually  the 
development  of  blood-vessels  progresses  from  the  region  of  the  area  opaca  into 
the  region  of  the  area  pellucida  and  extends  into  the  body  of  the  embryo.  We 
even  have  the  embryo  almost  completely  surrounded  by  a  region  of  extra-em- 
bryonic blood-vessels — the  definitive  area  vasculosa.  Now,  it  will  be  remem- 
bered that  the  area  opaca  is  the  territory  in  which  the  entodermal  cells  are 
actively  assimilating  the  yolk,  and  we  must  believe  that  the  blood-vessels  which 


THE  YOLK-SAC. 


89 


are  thus  early  developed  in  close  contact  with  the  cells  of  this  area  are  destined 
to  take  up  food  material  digested  by  the  entodermal  cells  and  carry  it  to  the 
embryo.  Hence  we  interpret  the  early  development  of  the  extra-embryonic 
vessels  as  due  to  physiological  necessities. 

The  mesoderm  at  first  forms  a  very  thin  layer  over  the  angioblast.  It  next 
thickens  by  the  multiplication  of  its  cells,  and  we  can  then  distinguish  in  it  both 
the  outer  mesothelium  and  the  inner  mesenchyma.  The  mesothelium  is  the 
permanent  external  cover  of  the  yolk-sac.  The  mesenchyma  grows  in  between 
the  primitive  blood-vessels,  and  finally  penetrates,  at  least  in  part,  between  the 
blood-vessels,  and  the  entoderm  of  the  yolk-sac,  a  condition  which  is  reached 
very  early  in  the  human  embryo  (Fig.  37). 

The  human  yolk=sac  is  characterized  by  its  small  size  and  by  the  precocious 


FIG.  39. — HUMAN  EMBRYO  OF  2.6  MM. — (After  W.  His.} 


expansion  of  the  area  vasculosa,  so  that  in  the  very  earliest  stage  known  to  us  by 
observation  blood-vessels  are  found  over  the  entire  sac.  At  the  beginning  of 
the  third  week  the  diameter  of  the  yolk-sac  is  about  equal  to  the  length  of  the 
embryo  (Fig.  65).  By  the  end  of  the  third  week  the  sac  has  become  distinctly 
pear-shaped,  its  narrower  pointed  end  being  that  by  which  it  is  connected  with 
the  intestinal  canal  of  the  embryo  (Figs.  38,  39).  The  sac  continues  growing, 
up  to  the  end  of  the  fourth  week,  after  which  it  enlarges  very  slightly,  if  at  all. 
Its  diameter  is  only  from  7  to  1 1  mm.  It  is  then  a  pear-shaped  vesicle  attached 
by  a  long  stalk  to  the  intestine,  the  stalk  having  been  formed  by  the  lengthen- 


90 


THE  EARLY  DEVELOPMENT  OF  MAMMALS. 


ing  of  the  neck  of  the  yolk-sac  (Fig.  40).     The  cavity  of  the  stalk  early  becomes 
obliterated  and  the  entoderm  in  the  stalk  disappears  altogether. 

The  Origin  of  the  Blood=vessels  and  Blood. 

As  stated  above  (pages  88  and  89),  the  first  blood-vessels  appear  in  the 
circumscribed  region  in  the  mesoderm  of  the  yolk-sac  and  lie  close  against  the 
entodermal  cells  of  the  area  opaca.  The  region  which  they  occupy  is  termed  the 
area  vasculosa.  From  the  area  vasculosa  the  development  of  blood-vessels  ex- 
tends, as  stated,  across  the  area  pellucida  into  the  embryo.*  During  these  early 


FIG.  40.  — HUMAN  EMBRYO  OF  9.8  MM.     PROBABLE  AGE  THIRTY  DAYS.     X  5  diams. 

stages  the  only  blood-vessels  are  in  the  splanchnopleure.  After  their  formation 
has  extended  into  the  body  of  the  embryo,  it  spreads  into  the  somatopleure  also, 
which,  therefore,  acquires  its  blood-vessels  at  a  later  stage.  It  should  be  noted, 
however,  that  the  development  of  the  blood-vessels  begins  before  the  ccelom  has 
been  developed  over  the  area  vasculosa.  While  they  are  forming,  the  coelom 
expands;  and  after  it  has  appeared,  the  primitive  blood-vessels  are  found  always 
exclusively  in  the  splanchnic  mesoderm. 

Definition.—  The  essential  part  of  a  blood-vessel  is  its  endothelial  wall.     In 

early  stages  all  the  blood-vessels  consist  only  of  endothelium.     Arteries  and 

* 

*  It  has  been  recorded  that  in  liz.irds  the  vascular  anlages  appear  first  in  the  area  pellucida. 


THE   ORIGIN  OF  THE  BLOOD-VESSELS  AND  BLOOD.  91 

veins  differ  but  little,  if  at  all,  in  histological  structure  during  early  embryonic 
stages,  and  are  distinguished  chiefly  by  the  direction  of  blood-currents  passing 
through  them.  Capillary  blood-vessels  and  sinusoids  have,  as  a  rule,  throughout 
life  merely  an  endothelial  wall.  Arteries  and  veins  become  strengthened  by  the 
development  of  special  coats  around  the  endothelium  which  arise  by  transforma- 
tions of  the  mesenchymal  cells  in  the  immediate  neighborhood  of  the  vessels. 

The  Development  in  the  Chick. — The  first  indication  of  the  blood-vessels  is  a 
reticulate  appearance,  which  can  be  recognized  in  the  mesoderm  in  surface  views 
of  the  fresh  or  hardened  embryo  at  the  end  of  the  first  day.  The  reticulate 
structure  increases  rapidly  in  extent  and  distinctness  during  the  second  day  of 
incubation.  It  is  confined  to  the  region  of  the  mesoderm  surrounding  the  em- 
bryo proper,  and  which  is,  therefore,  known  as  the  area  vasculosa,  as  above 
stated.  As  soon  as  there  are  several  primitive  segments  in  the  embryo,  the  net- 
work in  the  mesoderm  shows  traces  of  coloration  in  irregularly  shaped  reddish- 
yellow  spots,  which  are  largest  and  most  numerous  around  the  caudal  end  of  the 
embryo.  These  spots  are  called  blood-islands  because  the  cells  in  them  are 
transformed  into  the  first  blood-corpuscles.  The  network  appearance  is  due  to 
the  development  of  the  angioblast,  which  is  a  set  of  cells  delaminated  from  the 
ectoderm  or  the  yolk,  and  intervening  between  the  mesoderm  proper  and  the 
entoderm.  The  angioblast  at  first  assumes  the  form  of  more  or  less  solid  cords. 
The  meshes  of  the  angioblast  are  partly  or  wholly  filled  by  mesodermic  cells. 
The  coelom  now  appears  in  the  extra-embryonic  area,  and  thereafter  the  anlages 
of  the  blood-vessels  are  connected  with  the  splanchnic  mesoderm  only.  The 
anlages  of  the  blood-vessel  at  this  stage  form  a  thick  network  without  distinction 
of  stem  or  branch,  except  that  the  edge  of  the  area,  bounded  by  a  broad  band  of 
angioblast,  gives  rise  to  a  single  large  vessel,  which  is  known  as  the  sinus  ter- 
minalis.  The  anlages  are  all  in  one  layer,  none  overlying  the  others,  and  up  to 
this  stage  they  are  all  solid.  The  terminal  sinus  becomes  connected  with  the 
venous  system. 

The  blood-islands  are  spots  where  there  is  a  cluster  of  cells,  which  remain 
attached  to  one  another  and  to  the  walls  of  the  vessels.  The  cells  develop  hemo- 
globin in  their  interior,  hence  the  clusters  have  a  reddish  color  which  renders  the 
islands  very  conspicuous  in  surface  views  of  fresh  specimens.  Blood-islands 
appear  first  in  the  area  opaca,  but  almost  immediately  after  in  the  pellucida  also. 
They  have  at  first  a  rounded  or  branching  form.  In  the  inner  part  of  the  latter 
they  are  small  and  stand  alone.  Toward  the  periphery  they  are  larger,  closer 
set,  and  more  united  with  one  another.  Their  development  is  greater  around 
the  caudal  end  of  the  embryo. 

In  the  next  stage  the  vascular  anlages  become  hollow,  and  then  may  be 


92 


THE  EARLY  DEVELOPMENT  OF  MAMMALS. 


called  true  blood-vessels.  When  they  acquire  a  lumen,  the  blood-islands  are 
found  to  remain  attached  usually  to  the  upper  side  of  the  vessel  like  a  thickening 
of  its  wall  (Fig.  41,  bl.  is}.  Very  soon  after  the  vessels  have  become  hollow  the 
cells  of  the  blood-islands  break  apart  and  lie  free  in  the  cavity  of  the  vessel,  thus 
forming  the  first  blood-corpuscles.  They  are  characterized  by  having  a  rounded 
nucleus  with  a  very  distinct  nucleolus,  and  a  minimal  covering  of  protoplasm 
only.  After  the  cells  have  become  free  the  amount  of  protoplasm  in  each  cell 
increases.  The  cells  multiply  rapidly  by  mitotic  division.  According  to  the 
prevalent  hypothesis,  all  of  the  colored  blood-corpuscles  are  descendants  of  these 
cells  derived  from  the  blood-islands. 

The  angioblast  continues  growing  by  the  development  of  buds  from  the 


bl.is 


En 


FIG.  41. — SECTION  OF  THE  AREA.  VASCULOSA  OF  A  CHICK  EMBRYO  OF  THE  SECOND  DAY. 
Som,   Somatopleure.     Spt,  Splanchnopleure.     EC,  Ectoderm.     En,  Entoderm.       bl.is,   Blood-islands.      V,    V, 

Blood-vessels.      X  227  diams. 


vessels  already  formed.  These  buds  are  rounded  or  pointed,  forming,  as  it  were, 
spurs.  They  often  end  by  meeting  one  another  and  uniting.  They  are  usually 
hollow  from  the  first,  and  after  they  meet  one  another  or  an  adjacent  vessel,  the 
cavities  become  continuous,  and  thus  the  vascular  network  is  extended. 

The  Development  in  Mammals. — The  origin  of  the  blood-vessels  in  mam- 
mals is  not  adequately  known.  The  solid  primary  anlages  appear  in  the  extra- 
embryonic  area  vasculosa  and  extend  later  into  the  embryo.  They  present  well- 
marked  blood-islands,  which  make  their  first  appearance  in  rabbit  embryos  of 
the  eighth  day,  just  before  the  appearance  of  the  first  primitive  segments.  It  is 
characteristic  of  mammals  that  the  entire  yolk-sac,  probably  owing  to  its  small 
size,  becomes,  very  early  indeed,  vascularized  throughout. 

The  Growth  of  the  Vessels  into  the  Embryo. — The  entrance  of  the  vessels  into 


THE  BLOOD-CORPUSCLES.  93 

the  embryo  chick  begins  toward  the  end  of  the  second  day.  The  buds  which 
form  the  extra-embryonic  angioblast  grow  first  toward,  then  into,  the  embryo. 
The  penetrating  vessels  follow  certain  prescribed  paths.  Part  of  the  vessels  run 
along  the  posterior  edge  of  the  amniocardiac  vesicles,  and  enter  into  connection 
with  the  posterior  end  of  the  heart,  which  has  meanwhile  been  developing,  and 
which — owing  to  the  early  separation  of  the  head  end  of  the  embryo  from  the  yolk 
—is  the  only  part  of  the  heart  which  the  vessels  can  reach  directly.  While  the 
vessels  are  approaching  the  heart  their  differentiation  into  various  sizes  is  going 
on,  the  smallest  ones  to  remain  as  capillaries,  the  larger  ones  to  become  arteries 
or  veins.  The  only  two  veins  in  the  first  stage  are  those  above  mentioned,  which 
are  called  the  ompnafo-mesaraic.  Another  set  of  vessels  penetrates  along  the 
sphlanchnopleure  of  the  body  on  each  side  until  they  attain  the  small  space 
between  the  notochord  and  myotome  and  the  entoderm,  where  they  fuse  so  as 
to  form  a  longitudinal  vessel,  the  anlage  of  the  descending  aorta.  It  should  be 
noted  that  this  anlage  is  primitively  double.  The  aorta  appears  first  in  the 
region  toward  the  head.  It  grows  forward  above  the  pharynx,  bends  ven- 
trally  just  behind  the  mouth,  dividing  as  it  bends,  one  branch  going  around 
each  side  of  the  future  pharynx  and  uniting  again  on  the  ventral  side  of  the  pha- 
rynx in  the  median  ventral  line,  in  order  to  join  the  anterior  end  of  the  tubular 
heart.  The  heart  begins  to  beat  before  the  vessels  unite  with  it.  The  first 
blood-cells  have  already  been  formed;  hence  as  soon  as  union  is  accomplished 
the  blood  circulation  starts  up,  the  blood  passing  through  the  aorta  to  the  body, 
thence  by  numerous  lateral  branches  to  the  area  vasculosa,  and  returning  by  the 
two  omphalo-mesaraic  veins  to  the  heart.  It  will  thus  be  seen  that  almost  the 
entire  circulation  is  extra-embryonic. 

The  other  embryonic  blood-vessels  are  developed  by  buds  from  the  walls  of 
the  vessels  already  present  in  the  embryo,  in  the  same  general  manner  as  new 
vessels  are  formed  in  the  area  vasculosa.  These  buds  give  rise  to  the  endothe- 
lium  only  of  the  embryonic  vessels.  When  a  vessel  becomes  an  artery  or  a  vein, 
the  media  and  adventitia  are  added,  as  above  stated,  by  differentiation  of  the 
surrounding  mesenchyma. 

The  " vasof ormative  cells"  of  Ranvier  are  probably  degenerating  blood- 
vessels and  not  the  anlages  of  vessels,  as  Ranvier  assumed. 

The  Blood=corpuscles. 

The  red  blood-cells  are  the  only  elements  contained  in  the  blood  during  the 
earliest  stages  of  the  vertebrate  embryo.  When  the  circulation  begins,  the  num- 
ber of  corpuscles  is  small,  but  rapidly  increases  by  division  of  the  corpuscles. 
The  cells  in  amniota  are  at  first  round;  in  the  chick  they  are  from  8.3  to  12.5  ,«. 


94  THE  EARLY  DEVELOPMENT  OF  MAMMALS. 

The  cells  are  at  first  granular  and  slightly  colored.  As  the  protoplasm  increases 
about  each  nucleus  the  cell-body  becomes  more  distinct,  more  colored  with 
hemoglobin,  and  more  homogeneous. 

By  examining  the  blood  of  chick  embryos  of  successive  ages  we  can  trace 
the  differentiation  of  the  red  cells.  We  find  that  the  protoplasm  enlarges  for 
several  days,  and  that  during  the  same  time  there  is  a  progressive  diminution 
in  the  size  of  the  nucleus,  which,  however,  is  completed  before  the  area  of  proto- 
plasm reaches  its  ultimate  size.  The  nucleus  is  at  first  granular,  and  its  nucleo- 
lus  or  nucleoli  stand  out  clearly.  As  the  nucleus  shrinks,  it  becomes  round  and  is 
colored  darkly,  and  almost  uniformly,  by  the  usual  nuclear  stains.  The  blood- 
cells  of  mammals  pass  through  the  same  metamorphosis  as  those  of  birds.  For 
example,  in  rabbit  embryos  of  eight  days  (Fig.  42,  A}  the  cells  have  reached 
the  stage  with  a  granular  nucleus  and  well-developed  cell-body.  Corpuscles  of 
this  kind  are  characteristic  of  fishes  and  amphibia,  and  this  may,  therefore,  be 
designated  as  the  ichthyopsidan  stage.  Two  days  later  the  nucleus  is  already 
smaller,  and  by  the  thirteenth  day  has  shrunk  to  its  final  dimensions.  This 
condition  of  the  corpuscles  is  characteristic  of  the  reptiles  and  birds,  and  maybe 
designated,  therefore,  as  the  sauropsidan  stage.  The  nucleated  stage  of  the  cells 
is  typical  of  embryonic  life  only  in  mammals.  During  the  fetal  period  the  nuclei 
of  the  red  cells  gradually  disappear  and  the  cells  are  transformed  into  the  non- 
nucleated  corpuscles,  which  occur  only  in  mammals,  so  that  this  last  may  be 
designated  as  the  mammalian  stage.  The  successive  stages  of  the  blood-corpus- 
cles in  mammals  illustrate  the  law  of  recapitulation  (page  41).  There  has  been 
much  discussion  as  to  the  manner  in  which  the  nucleus  disappears  in  order  to 
convert  the  nucleated  cell  into  the  non-nucleated  mammalian  blood-corpuscles, 
but  authorities  are  not  yet  agreed.  When  the  nucleus  disappears,  the  corpuscle 
becomes  smaller.  In  the  human  embryo  at  one  month,  the  red  cells  are  the  only 
blood-corpuscles.  At  two  months  they  are  still  the  most  numerous,  although 
the  non-nucleated  corpuscles  have  begun  to  appear.  At  three  months  the  non- 
nucleated  corpuscles  constitute  by  far  the  majority  of  all  corpuscles  in  the  blood. 

Leucocytes. — The  origin  of  the  first  leucocytes  in  the  embryo  is  still  uncertain. 
Blood  is  found  to  contain  for  some  time  only  the  red  cells,  the  leucocytes  not 
appearing  in  the  chick  until  about  the  eighth  day,  in  the  rabbit  about  the  ninth, 
and  in  the  elasmobranchs  not  until  the  embryo  is  well  advanced  in  its  develop- 
ment. It  is  generally  believed  that  the  leucocytes  do  not  arise  in  the  blood- 
vessels, and  that  they  have  no  genetic  relationship  to  the  red  blood-corpuscles. 
It  is  probable  that  leucocytes  are  of  several  kinds,  and  of  several  distinct  origins. 


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96 


THE  EARLY  DEVELOPMENT  OF  MAMMALS. 


The  Origin  of  the  Heart. 

At  about  the  time  blood-vessels  are  developing  in  the  area  vasculosa,  the 
head  of  the  embryo  is  found  to  have  grown  so  much  that  it  projects  forward 
somewhat.  This  process  is  illustrated  by  the  diagrams  in  figure  43,  which  are 
intended  to  illustrate  the  method  by  which  the  so-called  separation  of  the  em- 
bryo and  yolk  takes  place.  It  is  often  said  that  the  embryo'  is  constricted  off 
from  the  yolk.  This,  however,  is  incorrect.  The  size  of  the  connection  between 
the  embryo  and  the  yolk  remains  absolutely  about  the  same,  or  even  increases  in 
dimensions,  but  the  embryo  grows  rapidly,  so  that  its  head  end  projects  forward 
and  later  its  caudal  end  also,  hence,  though  the  connection  with  the  yolk  may 
remain  unchanged,  the  growth  of  the  embryo  causes  that  connection  to  appear 
relatively  smaller.  In  early  stages  the  head  of  the  embryo  grows  more  rapidly 


FIG.  43. — DIAGRAMS  TO  ILLUSTRATE  THE  SEPARATION  OF  THE  EMBRYO  FROM  THE  Y6i.K. 
bl,  Blastopore.     /i,  Head  of  embryo.     Ach,  Archenteron  or  entodermal  cavity,     ec,  Ectoderm. 


than  the  caudal  end  (Fig.  43,  C).  At  the  cervical  end  of  the  anlage  of  the  head, 
where  the  tissues  of  the  embryo  bend  over  to  join  the  yolk,  a  portion  of  the  ccelom 
is  early  developed.  It  extends  across  the  median  line.  This  coelom  is  the  be- 
ginning of  the  pericardial  cavity.  In  connection  with  it  the  development  of  the 
heart  occurs.  The  formation  of  this  organ  is  probably  initiated  by  an  ingrowth 
of  the  future  cells  of  the  angioblast,  which  give  rise  to  the  endothelium  of  the 
heart.  The  mesothelium  of  the  dorsal  side  of  the  primitive  pericardial  coelom 
produces  the  muscular  walls  of  the  heart.  The  early  development  and  primi 
tive  relations  of  this  organ  can  be  understood  by  the  account  given  in  the  prac- 
tical part  of  the  structure  of  a  chicken  embryo  with  seven  segments. 


THE  MAIN  VESSELS  OF  THE  AREA    VASCULOSA.  97 

The  Germinal  Area. 

The  germinal  area  is  that  portion  of  the  amniote  ovum  (mammalian  blasto- 
dermic  vesicle)  in  the  center  of  which  the  embryo  is  differentiated.  It  com- 
prises, therefore,  both  the  embryo  proper  and  the  region  immediately  surround- 
ing it.  In  its  center  we  find  the  anlages  of  the  embryonic  structures  proper.  In 
its  extra-embryonic  part  we  find  the  three  primitive  germ-layers.  Underneath 
the  entoderm  is  the  cavity  of  the  yolk-sac.  In  the  mesoderm  we  have  occurring 
the  development  of  the  coelom,  and  in  the  somatic  mesoderm  the  differentiation  of 
the  primitive  blood-vessels.  These  primitive  vessels  occupy  the  sharply  defined 
territory,  the  edge  of  which  is  marked  by  the  sinus  terminalis.  The  first  differen- 
tiation in  the  germinal  area  which  can  be  clearly  recognized  by  the  naked  eye  is 
the  appearance  of  the  area  pellucida,  which  is  due  to  the  thinning  of  the  entoderm 
over  the  central  area.  Next  ensues  the  differentiation  of  the  primitive  streak. 
Further  progress  results  in  the  gradual  differentiation  of  the  embryo,  in  the 
sharp  demarcation  of  the  area  pellucida,  which  becomes  pear-shaped,  and  in  the 
appearance  of  the  blood-vessels  and  the  resulting  differentiation  of  the  area 
vasculosa.  Figure  167,  on  page  296,  represents  the  embryonic  area  of  a  hen's 
ovum  after  about  twenty-seven  hours'  incubation.  The  embryo  is  well  advanced 
in  development,  for,  although  the  primitive  streak,  pr,  still  remains  in  part  and 
the  medullary  groove  is  still  open  behind,  the  brain  is  already  marked  out  and 
the  head  has  become  partly  free.  Alongside  the  medullary  canal  lie  eight  pairs  of 
segments.  Around  the  embryo  one  easily  recognizes  the  somewhat  pear-shaped 
area  pellucida,  A.  p,  and  the  darker  area  opaca,  A.  o,  by  which  it  is  enclosed. 
The  area  vasculosa  stands  out  conspicuously  and  is  bounded  by  the  already  dis- 
tinguishable sinus  terminalis,  st.  Around  and  underneath  is  the  translucent  pro- 
amnion,  pro.  am,  from  which  the  mespderm  is  altogether  absent,  and  which^ 
therefore,  cannot  contain  any  blood-vessels.  Nor  are  there  at  this  state  any 
vessels  in  front  of  the  pro-amnion. 

The  Main  Vessels  of  the  Area  Vasculosa. 

Soon  after  the  capillary  network  of  the  areas  opaca  and  pellucida  has  pene- 
trated the  embryo,  certain  lines  of  the  network  begin  to  widen,  and  soon  dis- 
tinctly assume  the  size  and  functions  of  main  trunks ;  some  of  these  unite  with 
the  posterior  venous  end  of  the  heart,  which  has  meanwhile  been  formed  in  the 
embryo,  and  others  become  connected  with  the  anterior  or  aortic  end;  even 
before  this  the  heart  has  begun  to  beat,  so  that,  as  soon  as  all  connections  are 
made,  the  primitive  circulation  starts  up.  The  arrangement  of  the  vessels  is 
not  the  same  in  birds  and  mammals,  although  commonly  so  stated.  The  dis- 
position in  birds  is  indicated  by  the  diagram  shown  in  figure  44,  in  which,  it 

7 


98 


THE  EARLY  DEVELOPMENT  OE  MAMMALS. 


should  be  remembered,  the  embryo  and  the  capillary  network  are  drawn  many 
times  too  large  in  proportion  to  the  area  vasculosa.  The  area  is  bounded  by  a 
broad  circular  vessel,  the  sinus  terminalis,  5.  T.,  which  constitutes  a  portion  of 
the  venous  system  in  birds,  for  in  front  of  the  head  of  the  embryo  the  sinus  leaves 
a  gap,  and  is  reflected  back  along  the  sides  of  the  body  of  the  embryo  to  make 
two  large  veins,  which,  after  uniting  with  the  other  venous  channels  coming 
from  various  parts  of  the  area  vasculosa  on  each  side,  enter  the  embryo  as  two 
large  trunks,  Om.  V.,  known  as  the  omphalo-mesaraic  veins;  these  two  veins 
unite  in  a  median  vessel,  the  sinus  venosus,  S.  V .,  which  runs  straight  forward 


P.O, 


AO 


1.T- 


Ora.A. 


Om 


FIG.  44. — DIAGRAM  OF  THE  CIRCULATION  IN  A  CHICK  AT  THE  END  OF  THE  THIRD  DAY,  AS  SEEN  FROM 

THE  UNDER  (ENTODERMAL)  SIDE. 

The  embryo,  with  the  exception  of  the  heart,  is  dotted ;  the  veins  are  black.  Ao,  Aorta.  Arc,  Aortic  arches. 
card,  Cardinal  vein.  D.  C,  Duct  of  Cuvier.  Ht,  Heart.  Jug,  Jugular  vein.  Om.  A,  Omphalo-mesaraic  or 
vitelline  artery.  Om.  V,  Omphalo-mesaraic  or  vitelline  vein.  S.  7\  Sinus  terminalis.  S.  V,  Sinus  venosus. 

and  enters  the  posterior  end  of  the  heart.  The  sinus  venosus  also  receives  the 
veins  from  the  body  of  the  embryo,  namely,  the  jugulars,  Jug.,  and  cardinals, 
card.;  the  former  from  in  front  unite  each  with  the  cardinal  of  the  same-  side, 
making  a  short  transverse  trunk  known  as  the  ductus  Cuvieri,  D.  C.;  the  two 
ducts  empty  into  the  sinus  venosus.  The  entire  venous  current  is  thus  brought 
to  the  heart  in  a  united  stream;  it  passes  out  through  the  aorta,  the  greater  part 
ascends  the  aortic  arches  and  passes  back  as  shown  in  the  figure,  Ao.,  and  divides 
at  the  posterior  fork  of  the  aorta,  the  bulk  of  the  two  currents  passing  out 


THE  MAIN  VESSELS  OE  THE  AREA    VASCULOSA. 


99 


through  omphalic  arteries,  Om.  A.,  and  thence  to  the  capillaries  of  the  area  vas- 
culosa  and  so  on  to  the  venous  trunks  again.  As  shown  in  the  figure,  which  pre- 
sents the  under  .side  of  the  area,  the  left  omphalo-mesaraic  vein  preponderates, 
and  in  the  latter  stages  this  difference  becomes  more  marked,  until  finally  the 
right  stem  is  very  inconsiderable  in  comparison  with  the  great  left  vein.  The 
time  at  which  the  disparity  commences  is  extremely  variable,  as  is  also  the  de- 
gree of  inequality  between  the  two  veins. 

The  following  description  probably  represents  what  was  the  primitive  con- 
dition of  vessels  in  the  mammalian  area  vasculosa.     It  applies  to  an  early  stage 


FIG.  45. — AREA  VASCULOSA  OF  A  RABBIT,  PRESUMABLY  OF  ABOUT   TWELVE   DAYS. — (After  Van  Beneden 

and  Jitlht.} 


in  the  rabbit,  which  has  been  figured  by  Bischoff,  whose  figure  is  copied  in  Kolli- 
ker's  "  Grundriss."  An  essentially  similar  arrangement  of  the  vessels  exists  also 
at  a  corresponding  stage  in  the  dog.  The  veins  are  much  more  symmetrical 
than  in  the  chick,  and  have  the  same  general  plan;  the  sinus  terminalis  belongs 
to  the  venous  system,  so  that  the  connection  with  the  arterial  circulation,  found 
later,  is  secondary ;  the  aorta  of  the  embryo  is  double,  and  gives  off  on  each  side 
(segmentally  arranged?)  transverse  branches,  one  of  which  develops  into  the 
large  trunk  shown  in  figure  45 ;  the  network  of  small  vessels  forms  two  layers,  of 


100  THE  EARLY  DEVELOPMENT  OF  MAMMALS. 

which  the  upper  is  connected  with  the  arteries,  the  lower  with  the  veins.     The 
change  from  the  earlier  condition  to  the  later  has  still  to  be  followed. 

According  to  Van  Beneden's  recent  researches  on  the  rabbit,  the  arrange- 
ment of  the  main  vessels  in  the  area  vasculosa  at  a  later  stage  is  quite  different. 
The  sinus  terminalis  forms  a  complete  ring  (Fig.  45),  and  is  connected  with  the 
arterial  system  by  a  single  trunk,  which  corresponds  to  the  left  omphalic  artery 
of  the  bird.  For  some  time  the  connection  between  the  embryonic  arteries  and 
the  area  vasculosa  is  entirely  through  capillaries,  and  the  arterial  trunk  on  the 
vascular  area  does  not  appear  in  the  rabbit  for  several  days.  There  are  two 
veins,  one  arising  from  each  side  of  the  body  and  passing|put  on  to  the  area  vascu- 
losa over  the  back  of  the  embryo;  they  are  the  two  large  upper  vessels  in  the 
figure.  •:  fc 

•  • 
The  Liver. 

When  the  omphalo-mesaraic  veins,  the  first  large  veins  to  appear,  are  devel- 
oped, they  are  situated  in  the  splanchnopleure  and  join  the  heart.  They  are  of 
such  large  size  as  to  cause  a  projection  into  the  coelom.  This  projection  is  the 
septum  transversum  (p.  82).  As  shown  in  the  diagram  (Fig.  43),  the  entoderm 
of  the  digestive  canal  of  the  head  of  the  embryo  passes  over  behind  the  pericardial 
cavity  and  behind  the  septum  transversum  into  the  yolk-sac.  Out  of  the  ento- 
derm covering  the  septum  transversum  on  its  caudal  side,  the  anlage  of  the  liver 
is  developed.  This  anlage  is  produced  by  a  rapid  proliferation  of  the  entodermal 
cells,  and  they  grow  toward  the  space  occupied  by  the  omphalo-mesaraic  veins. 
An  intergrowth  of  the  liver  cells  and  of  the  endothelium  of  the  veins  takes  place. 
The  cavity  of  the  veins  becomes  subdivided  into  smaller  blood  channels  which 
we  call  sinusoids  to  distinguish  them  from  capillary  vessels.  The  liver  cells 
arrange  themselves  in  the  form  of  cords  which  are  termed  the  hepatic  cylinders. 
Each  hepatic  cylinder  is  closely  invested  by  the  venous  endothelium.  The  liver 
consists  at  first  only  of  hepatic  and  endothelial  cells  and  is  situated  in  the  septum 
transversum. 

When  the  liver  becomes  larger,  it  protrudes  from  the  septum  transversum, 
but  does  not  separate  from  it,  so  that  in  the  adult  the  liver  is  always 'found  at- 
tached to  the  diaphragm,  which  is  merely  the  modified  septum  transversum. 

The  Oral  and  Anal  Plates. 

These  two  structures  resemble  one  another.  Each  occupies  a  small  area 
and  is  formed  by  the  intimate  union  of  the  entoderm  with  the  ectoderm.  When 
the  union  is  first  formed  the  two  layers  are  distinct,  but  they  soon  fuse,  so  that  no 
boundary  can  be  recognized  between  them .  Ultimately  both  plates  break  down , 


THE  EXCRETORY  ORGANS. 


101 


their  cells  disappearing,  and  they  are  replaced  by  openings,  that  of  the  oral  plate 
forming  the  opening  between  the  mouth-cavity  and  the  pharynx,  that  of  the  anal 
plate  forming  the  primitive  anal  opening.  The  anal  plate,  before  it  breaks  down, 
makes  a  considerable  growth,  forming  an  epithelial  mass  which  plays  an  im- 
portant part  in  the  anatomical  modeling  of  the  region.  The  oral  plate  disap- 
pears very  early;  the  anal  plate  much  later. 

As  soon  as  the  head  of  the  embryo  has  grown  so  much  as  to  project  as  an 
independent  part,  we  find  that  the  oral  plate  lies  on  the  under  surface  of  the 
head,  a  little  in  front  of  the  heart.  The  pro-amnion,  pro.  am.,  arises  from 
the  somatopleure  enclosing  the  heart,  ht.,  so  that  when  the  oral  plate  be- 
comes perforate,  the  cavity  of  the  entoderm,  Ent.,  will  communicate  directly 
with  the  cavity  enclosed  by  the  pro-amnion,  or,  in  other  words,  with  the  per- 
manent amniotic  cavity. 

A  similar  anal  plate  at  the  posterior  end 
of  the  embryo  also  lies  within  the  amnion 
(Fig.  46).  This  figure  is  taken  from  a  sheep 
embryo  in  a  very  early  stage,  so  that  the 
anal  plate  appears  to  lie  on  the  dorsal  side. 
By  the  curling  ventralwards  or  the  bending 
over  of  the  tail  end  of  the  young  embryo  the 
anal  plate  is  gradually  transferred  or  rolled 
over  on  to  the  ventral  side,  where  it  per- 
manently remains. 


Ssfe  Amn 


FIG.  46. — LONGITUDINAL  SECTION  OF  THE 
POSTERIOR  END  OF  A  SHEEP  EMBRYO 
OF  SIXTEEN  DAYS. 

Amn,  Amnion.  a.m,  Anal  membrane  (or 
plate),  pr.s,  Primitive  streak.  En,  Ento- 
derm. Ach,  Archenteron,  or  entodermal 
cavity  of  the  embryo.  All,  Anlage  of  al- 
lantois.  mes,  Mesoderm. — (After  R. 
Bonnet. ) 


The  Excretory  Organs. 

No   less  than   three   distinct   excretory 
organs  are  known  to  occur  in  vertebrates. 

Of  these,  the  first  is  termed  the  pronephros,  or  head  kidney,  on  account  of 
its  position  toward  the  head  and  in  the  neighborhood  of  the  heart.  It  is  well 
developed  and  the  only  excretory  organ  in  many  fishes  and  in  the  larval  stages  of 
amphibia.  In  elasmobranchs,  which  occupy  in  this  respect  an  exceptional  posi- 
tion, and  in  amniota  it  exists  in  a  rudimentary  form  only,  except  as  to  its  duct, 
which  plays  an  important  role  in  the  further  development.  The  pronephros  con- 
sists of  a  few  epithelial  tubes  which  take  a  somewhat  twisting  course,  but  may  be 
said  to  run,  in  general  terms,  transversely.  Each  tube  begins  with  a  ciliated 
funnel-shaped  opening  (Fig.  47,  /)  not  far  from  the  median  line  of  the  embryo, 
and  ends,  after  a  more  or  less  contorted  course,  in  a  longitudinal  duct,  which, 
after  receiving  all  of  the  tubules,  runs  toward  the  posterior  end  of  the  embryo 
and  opens  into  the  extremity  of  the  entodermal  or  digestive  canal.  Opposite  the 


102 


THE  EARLY  DEVELOPMENT  OF  MAMMALS. 


funnels,  and  separate  from  the  pronephros  proper,  there  is  a  so-called  glomus 
(Fig.  47,  gl),  which  is  a  projection  of  not  inconsiderable  size  from  the  mesentery. 
When  fully  developed  the  glomus  contains  a  rich  network  of  blood-capillaries, 
so  that  it  somewhat  resembles  the  glomerulus  of  the  kidney.  The  circulation  of 
the  pronephros  is  sinusoidal. 

The  second  of  the  excretory  organs  is  termed  the  mesonephros,  Wolffian 
body,  or  foetal  kidney.  It  is  absent  in  many  fishes,  but  it  is  well  developed  in 
elasmobranchs.  In  adult  amphibians  it  replaces  the  pronephros,  which  is  purely 
a  larval  structure.  It  is  present  in  the  embryos  of  all  amniota,  but  undergoes  a 
partial  degeneration  before  adult  life,  being  itself  replaced  in  adult  amniota  by  the 
true  kidney.  The  mesonephros  resembles  somewhat  the  pronephros,  especially 


•nch 


FIG.  47. — FROG  (RANA  TEMPORARIA)  TADPOLE  OF  12.0  MM.     CROSS-SECTION  OF  THE  PRONEPHRIC  REGION. 

nch,  Notochord.     m,  Muscles,    f,  Pronephric  funnel,     v,  Blood-vessel.      EC,   Ectoderm,     t,  Pronephric  tubule. 

gl,  Glomus.      Lit,  Lung.     X  9°  diams. — (After  M.  Fiirbringer.} 

as  found  in  the  ichthyopsida.  It  occupies  a  much  larger  region  of  the  body  than 
the  pronephros.  It  has  no  glomus  associated  with  it,  but  each  tubule  contains 
a  glomerulus  very  similar  in  its  general  structure  to  the  glomerulus  of  a  true 
kidney.  In  the  ichthyopsida  each  tubule  begins  with  a  ciliated  funnel,  and,  after 
making  several  coils,  opens  into  the  pronephric  duct.  The  circulation  of  the 
organ  is  sinusoidal.  In  the  amniota  the  mesonephros,  or,  as  it  is  more  commonly 
called  in  these  animals,  the  Wolffian  body,  is  essentially  an  embryonic  structure. 
Its  tubules,  however,  do  not  have  at  any  stage  the  ciliated  funnels  to  be  found  in 
amphibia  and  fishes,  but  they  have  glomeruli  and  they  open  into  the  pronephric 
duct,  which,  on  account  of  its  relations  to  the  organs,  is  in  this  type  more  com- 
monly spoken  of  as  the  Wolffian  duct.  The  circulation  of  the  organ  is  sinusoidal. 


THE  ALLANTOIS.  103 

Further  details  are  given  in  the  practical  part  in  connection  with  the  study  of 
the  embryo  pig  and  chick.  , 

The  third  of  the  excretory  organs  is  termed  the  metanephros  or  true  kidney. 
It  exists  in  all  aclult  amniota,  but  only  in  them.  In  development  and  in  struc- 
ture it  differs  very  much  from  the  other  excretory  organs.  For  an  account  of  its 
origin  in  mammals,  see  page  2J#^  3  ^ 

It  is  essential  that  the  student  of  embryology  should  have  a  clear  prelimi- 
nary notion  of  these  organs,  for  without  such  he  will  be  unable  to  comprehend  an 
important  series  of  embryological  phenomena. 

The  Allantois. 

The  allantois  is  a  diverticulum  of  the  entodermal  canal,  and  is,  therefore, 
lined  by  entodermal  epithelium.  It  arises  on  the  ventral  side  of  the  caudal  end 
of  the  embryo  in  proximity  to  the  anal  plate.  In  its  development  we  can  dis- 
tinguish two  main  types.  The  first  type  is  illustrated  by  the  sauropsida  and  the 
ungulates.  In  them  it  grows  out  and  rapidly  enlarges  so  as  to  form  a  vesicle  of 
considerable  size  and  connected  with  the  embryo  by  means  of  a  narrow,  hollow 
stalk.  When  the  allantois  develops  according  to  this  type,  it  is  spoken  of  as  free, 
because  it  has  no  connection  with  the  extra-embryonic  somatopleure  (chorion 
and  amnion).  This  form  of  the  allantois  may  be  readily  observed  in  chicken 
embryos,  for  by  the  fourth  day  it  has  become  a  considerable  rounded  vesicle 
which  lies  in  the  extra-embryonic  coelom  between  the  yolk-sac  and  the  extra- 
embryonic  somatopleure  or  membrana  serosa.  During  the  fifth  day  it  rapidly 
enlarges,  and  at  the  beginning  of  the  sixth  day  is  nearly  or  quite  as  large  as  the 
head  of  the  embryo.  In  ungulates  the  growth  of  the  free  allantois  begins  very 
early  and  becomes  enormous.  Its  principal  expansion  is  sideways,  that  is  to  say, 
at  right  angles  to  the  axis  of  the  embryo,  and  it  becomes  a  large  sac,  very  much 
larger,  indeed,  than  the  entire  embryo. 

The  second  type  of  allantois  occurs  in  the  placental  mammals  of  the  unguic- 
ulate  series  and  is  not  known  to  occur  in  any  species  of  the  ungulate  type.  In 
probably  all  unguiculates  the  posterior  end  of  the  body  has  a  prolongation  which 
is  known  as  the  body-stalk.  Into  this  body-stalk  the  diverticulum  constituting 
the  allantois  extends.  The  entoderm  of  the  allantois  is  surrounded  by  mesoderm , 
which  is  present  in  the  body-stalk  in  considerable  volume.  On  the  outer  surface 
there  extends  a  layer  of  ectoderm,  so  that  the  three  germ-layers  enter  into  the 
formation  of  the  body-stalk  as  they  do  into  the  formation  of  the  embryo.  These 
relations  are  illustrated  by  the  diagram  (Fig.  48).  By  means  of  the  body- stalk 
a  connection  is  established  between  the  embryo  and  the  extra-embryonic  soma- 
topleure or  primitive  chorion,  Cho.  Later,  when  the  formation  of  the  amnion  is 


104 


THE  EARLY  DEVELOPMENT  OF  MAMMALS. 


Einb. 


Cho. 


completed,  the  essential  relations  are  found  to  be  as  illustrated  by  the  diagram 
(Fig.  48,  B).  The  amnion  arises  from  the  distal  end  of  the  body-stalk,  but  the 
body-stalk  retains  its  connection  with  the  chorion.  When  the  allantois  becomes 
free,  the  connection  with  the  chorion  is  entirely  lost.  The  maintenance  of  that 
primitive  connection  in  the  unguiculates  is  to  be  regarded  as  a  new  modification 
of  the  relations  of  the  embryonic  appendages,  evolved  only  in  the  higher  animals- 
The  maintenance  of  that  connection  makes  possible  the  modification  in  the 

structure  of  the  chorion,  which  is  of 
the  greatest  morphological  impor- 
tance. This  modification  is  the  de- 
velopment of  the  blood-vessels  in 
the  chorion.  The  anlages  of  these 
blood-vessels  are  outgrowths  of  the 
embryonic  angioblast.  They  appear 
so  as  to  form  four  vessels  which  grow 
through  the  length  of  the  body-stalk 
in  the  neighborhood  of  the  allantoic 
diverticulum.  Two  of  these  vessels 
are  veins  and  two  are  arteries.  They 
are  termed  the  umbilical  vessels.  The 
veins  at  the  embryonic  end  of  the 
body-stalk  enter  the  somatopleure 
of  the  embryo,  through  which  they 
make  their  way  toward  the  heart. 
The  umbilical  arteries,  where  they 
join  the  embryo,  are  found  to  unite 
and  join  the  main  aorta,  so  that  they 
maybe  termed  the  terminal  branches 
of  the  embryonic  aorta.  In  early 
stages  they  are  the  largest  branches 
which  the  aorta  has.  At  the  distal 
end  of  the  body-stalk  the  four  vessels 
enter  the  mesoderm  of  the  chorion, 

there  branch  abundantly,  and  produce  a  rich  network  of  blood-vessels  throughout 
the  entire  membrane.  The  unguiculate  mammals,  therefore,  are  characterized  by 
this  special  feature,  the  possession  of  the  body-stalk  which  contains  the  allantoic 
diverticulum  and  gives  access  for  the  blood-vessels,  and  therefore  also,  of  course, 
for  the  blood,  to  the  chorion,  which  thus  becomes  vascular.  In  all  other  amniota 
the  chorion  is  without  blood-vessels. 


All. 


Am. 


Emb. 


Cho. 


FIG.  48. — DIAGRAMS  ILLUSTRATING  THE  RELATIONS 
OF  THE  ALLANTOIS  IN  UNGUICULATE  MAMMALS. 

A,  Before,  B,  after  the  formation  of  the  amnion.  All, 
Entodermal  allantois.  Am,  Amnion.  b.s,  Body- 
stalk.  Cho,  Chorion.  Cos,  Extra-embryonic  coelom. 
Emb,  Anterior  end  of  embryo.  Yk,  Yolk-sac. 


THE  ALLANTOIS.  105 

The  size  of  the  allantoic  cavity  in  unguiculates  varies  considerably.  In  man 
it  is  minimal,  forming  only  a  long  and  very  narrow  tube  (compare  page  138).  In 
rodents  it  expands  somewhat,  but  it  never  becomes  free  in  the  sense  that  it  is 
separated  from  the  body-stalk,  although  it  may  acquire  a  partial  independence. 
In  this  case  it  may  also  become  more  or  less  vascular  by  the  development  of 
branches  from  the  umbilical  arteries  and  veins  around  the  allantois. 

In  those  animals  in  which  the  allantois  is  free,  the  umbilical  arteries  and 
veins  have  all  their  branches  in  the  allantois,  there  being  no  body-stalk.  The 
embryo  is  without  connection  with  the  chorion,  and,  therefore,  these  vessels  in 
their  ramifications  are  restricted  to  the  allantois. 

Relations  of  the  Allantois  to  the  Chorion  in  Ungulates. — Since  the  true  chorion 
is  the  outermost  of  the  foetal  envelopes,  it  alone  can  come  in  contact  with  the  walls 
of  the  uterus.  All  placental  developments,  therefore,  necessarily  depend  upon 
the  .chorion.  Now,  in  ungulates,  where  the  chorion  is  without  blood-vessels, 
there  is  no  physiological  apparatus  to  transfer  any  nutritive  material  which  may 
be  taken  up  by  the  chorion  from  the  uterus  to  the  embryo  until  a  second  union 
takes  place  between  the  vascularized  allantois  and  the  chorion.  The  inner  sur- 
face of  the  chorion  and  the  outer  surface  of  the  allantois  are  both  mesodermic. 
The  two  mesodermic  layers  come  into  contact  with  one  another  and  unite  loosely. 
The  vessels  of  the  allantoic  mesoderm  are  thus  brought  into  physiological  union 
with  the  chorion,  but,  being  allantoic  vessels,  they  are,  of  course,  morphologically 
different  from  the  chorionic  vessels  of  unguiculate  mammals.  These  considera- 
tions demonstrate  that  the  unguiculate  placenta  is  allantoic  rather  than 
chorionic,  and  is,  morphologically  speaking,  essentially  different  from  the  true 
chorionic  placenta,  which  can  be  developed  only  in  those  animals  and  embryos 
which  have  a  permanent  body-stalk. 

The  simple  relations  of  the  chorion  in  the  ungulata  to  the  uterine  wall  is 
illustrated  by  the  accompanying  figure  49,  which  shows  a  portion  of  the  chorion 
of  a  pig  embryo  of  15  mm.,  together  with  the  surface  of  the  uterus  to  which  it  was 
fitted.  The  two  membranes  were  accidentally  separated  in  the  preparation. 
The  chorion  consists  of  a  layer  of  cylinder  epithelial  cells,  EC,  each  of  which  can 
be  distinctly  made  out,  and  of  a  layer  of  mesoderm,  Mes,  containing  only  few 
cells  and  blood-vessels,  two  of  which,  Ve,  are  shown  in  the  section;  the  meso- 
dermic cells  are  a  little  more  crowded  near  the  epithelium.  Each  ectodermal 
cell  is  distinctly  marked  off  from  its  neighbors  by  a  line.  The  protoplasm  stains 
somewhat,  the  nuclei  are  slightly  oval  and  granular,  and  are  situated  near  the 
middle  of  the  cells.  The  top  of  each  cell  is  concave.  The  uterine  epithelium, 
Ut.  Ep,  resembles  in  the  general  form  of  its  cells  and  in  the  character  of  its  proto- 
plasm the  chorionic  ectoderm,  but  differs  from  it  in  that  each  cell  has  a  convex 


106 


THE  EARLY  DEVELOPMENT  OF  MAMMALS. 


free  end,  and,  further,  in  that  the  nuclei  of  the  cells  are  situated  near  the  top  of 
the  layer.  When  the  relations  of  the  two  epithelia  have  not  been  disturbed,  it  is 
readily  observed  that  the  concavity  of  each  chorionic  ectodermal  cell  receives  the 
convex  end  of  the  uterine  epithelial  cell,  so  that  the  two  layers  are  closely  fitted 
together,  cell  for  cell. 

The  Trophoblast. 

The  trophoblast  is  the  name  applied  to  the  special  layer  of  cells  developed 
on  the  outer  surface  of  the  ectoderm  of  the  mammalian  blastodermic  vesicle. 


Ut.Ep. 


Conn. 


FIG.  49. — PIG,  15.0  MM.,  SERIES   135,  SECTION   58,  TO  SHOW  THE  RELATIONS  OF  THE  CHORION  TO  THE 

UTERUS. 

Conn,  Connective   tissue  of  the  uterus.     £<:,  Chorionic  ectoderm.     Mes,  Chorionic  mesoderm.      Ut.Ep,  Uterine 
epithelium.      Ve,  Chorionic  blood-vessel.     X  35°  diams. 


It  has  as  yet  been  observed  only  in  unguiculates.  The  trophoblastic  layer  may 
be  developed  over  the  entire  surface  of  the  ovum,  or  over  only  a  portion  thereof. 
Its  principal  known  function  is  to  destroy  the  tissues  of  the  uterus  of  the  mother 
with  which  it  comes  in  contact.  The  destruction  of  the  tissue  is  supposed  to 
serve  two  purposes:  First,  to  supply  nutrition  to  the  embryo.  It  is  from  this 
supposed  function  that  the  layer  derives  its  name  of  trophoblast.  Second,  to 
secure  the  attachment  of  the  ovum  to  the  wall  of  the  uterus.  This  preliminary 
attachment  is  called  the  implantation  of  the  ovum.  In  some  cases  the  tropho- 
blast is  developed  very  early  over  the  surface  of  the  ovum,  appearing  almost  as 


THE  GROWTH  OF  THE  EMBRYO.  107 

soon  as  the  stage  of  the  blastodermic  vesicle  is  reached,  and  while  the  vesicle  is 
very  small.  In  such  cases  the  ovum  makes  a  cavity  for  itself  by  dissolving  away 
the  epithelium  and  connective  tissue  at  a  small  spot  on  the  uterine  surface,  mak- 
ing a  cavity  in  which  it  lodges  itself.  In  other  cases  the  trophoblast  is  developed 
later  and  does  not  appear  over  the  whole  of  the  blastodermic  vesicle.  The  area 
over  which  it  exists  in  such  cases  is  called  the  placental  area  (compare  pages 
121  and  122).  The  trophoblast  in  these  forms  unites  very  closely  indeed  with  the 
surface  of  the  uterus,  and  the  uterine  tissues  undergo  degeneration  and  resorp- 
tion.  We  may  regard  as  the  first  step  toward  the  production  of  the  placenta 
proper  the  disappearance  of  the  trophoblast.  Our  knowledge  of  its  disap- 
pearance is  incomplete,  but  it  is  probable  that  it  is  due  to  a  transformation  of  the 
cells  of  the  trophoblast,  associated  with  contemporaneous  modifications  of  the 
chorionic  membrane,  of  which  the  general  result  may  be  said  to  be  formation  of 
the  chorionic  villi  which  constitute  the  foetal  portion  of  the  placenta.  The 
modified  trophoblastic  cells  are  supposed  to  enter  into  the  formation  of  the 
ectodermal  covering  of  these  villi. 


The  Growth  of  the  Embryo. 

In  all  vertebrates  there  is  provision  made  for  the  nutrition  of  the  embryo, 
the  development  being  strictly  of  the  embryonic  type.  In  most  cases  this  pro- 
vision consists  in  a  sort  of  yolk  material,  but  in  the  placental  mammals  the  pro- 
vision is  made  by  means  of  the  placenta  from  the  uterus  of  the  mother.  In 
either  case  the  embryo  has  only  to  assimilate  the  food  which  is  already  more  or 
less  prepared  for  it,  and  we  find  that  in  all  vertebrates  there  is  an  extremely  rapid 
growth  of  the  embryo.  In  amniota  we  have  a  marked  distinction  between  the 
embryo  proper  and  its  so-called  appendages,  the  yolk-sac,  chorion,  amnion,  and 
allantois.  These  appendages  are  all  ultimately  sacrificed  for  the  benefit  of  the 
embryo,  and  in  mammals,  except  for  a  portion  of  the  allantois  which  is  retained 
within  the  body  of  the  embryo  as  the  anlage  of  the  bladder,  these  appendages  are 
ultimately  cast  off  altogether,  and  take  no  part  in  the  construction  of  the  child 
after  birth.  We  note,  in  fact,  as  we  ascend  the  vertebrate  series,  an  increasing 
tendency  to  give  the  embryo  prominence  and  differentiate  it  more  decisively 
from  the  embryonic  appendages.  This  becomes  so  marked  in  the  higher  verte- 
brates that  we  speak  of  the  growth  of  the  embryo  almost  as  a  separate  thing  from 
the  growth  of  the  appendages. 

The  embryo,  when  its  differentiation  commences,  lies  as  a  small  area  upon 
the  surface  of  the  ovum.  By  the  growth  of  the  tissues  of  this  embryonic  region, 
the  embryo  at  once  begins  to  enlarge,  and  as  it  enlarges  we  see  that  it  outstrips 
the  extra-embryonic  structures  with  which  it  is  associated,  and  first  the  head 


108 


THE  EARLY  DEVELOPMENT  OF  MAMMALS. 


end  of  the  embryo  becomes  so  large  as  to  rise  up  from  the  general  surface  of  the 
ovum  and  then  to  project  forward.  A  very  little  later  a  similar  process  occurs 
at  the  caudal  end,  and  the  whole  body  of  the  embryo  rises  now  above  the  yolk, 
and  the  further  growth  results  not  only  in  a  greater  protrusion  of  the  head  for- 


Op.L. 


To. 


epen. 


EC. 


bas.g. 


FIG.  50. — TRANSVERSE  SECTION  OF  AN  EMBRYO  CATFISH  (AMIURUS)  ;  SERIES  25,  SECTION  43. 
Ac,  Aorta,     bas.g,  Basal  ganglion    of  mid-brain.     EC,  Ectoderm,     epen,   Ependymal  layer  of  mid-brain,   it, 
Cavity  of  mid-brain.     Z,  Lens.     Mk,  Meckel's  cartilage.     N.op,  Optic  nerve.      Op.L,    Optic  lobe.     Per. 
cat,  Pericardial  ccelom.     PA,  Pharynx,     pig,  Pigment  layer  of  the  eye.     R,  Retina.      To,  Torus.      Trab, 
Trabecula  cranii.     x,  Undetermined  organ.      Yk,  Yolk.      X  4°  diams. 

ward  and  of  the  tail  backward,  but  also  of  the  body  sideways,  so  that  now  the 
embryo  appears  to  have  a  constricted  connection  with  the  rest  of  the  ovum.  The 
general  character  of  the  process  may  be  readily  understood  by  comparison  of  the 
three  diagrammatic  cross-sections  (Figs.  34, 3 1 ,  A ,  and  3 1 ,  B],  and  also  of  the  three 


THE   UMBILICAL   CORD.  109 

diagrammatic  longitudinal  sections  in  figure  43.  If  we  view  the  longitudinal 
sections,  we  see  an  increasing  protuberance  of  the  head  and  tail  ends  of  the  em- 
bryo, so  that  the  embryo  appears  more  and  more  separated  from  the  yolk.  The 
process  has  long  been  traditionally  described  as  a  folding-in  of  the  germ-layers, 
but  this  traditional  description  is  incorrect,  for  the  separation  of  the  embryo  is 
really  due  to  the  expansion  of  the  embryo,  not  to  the  constriction  of  this  connec- 
tion with  the  yolk.  The  diagrams  referred  to  show  at  a  glance  how  the  original 
width  of  the  communication  is  retained,  while  the  intestinal  canal  or  embryonic 
archenteron  extends  forward  and  backward.  In  figure  43,  A,  the  archenteron 
is  open  to  the  yolk  throughout  its  entire  extent.  In  B  the  head  has  begun  to  be 
free,  and  with  it  the  archenteric  cavity  has  begun  to  extend  forward  and  forms  a 
distinct  cephalic  portion,  which  is  entirely  within  the  embryo  and  is  not  open 
directly  to  the  yolk  or,  as  it  would  be  in  mammals,  into  the  entodermal  cavity  of 
the  blastoiermic  vesicle.  In  C  the  tail  also  has  grown  forth  from  the  yolk,  and 
the  archenteron  with  it,  so  that  now  we  have  a  caudal  embryonic  digestive  canal. 
By  further  development  the  embryo  enlarges  more  and  more,  but  the  opening 
into  the  yolk-sac  remains  nearly  the  same  absolute  size. 

The  relations  of  the  embryo  to  the  yolk  in  the  anamniota  are  illustrated  by 
the  accompanying  figure  50,  which  represents  a  transverse  section  through  a 
young  stage  of  the  catfish  (Amiurus).  The  section  passes  through  the  head  of  the 
embryo  and  shows  both  eyes  and  the  slender  optic  nerves,  N .op,  almost  symmet- 
rically cut  on  both  sides.  The  yolk,  Yk,  is  a  large  mass  heavily  laden  with  yolk- 
granules.  Between  the  tissues  of  the  embryo  proper  and  of  the  yolk-sac  there  is 
a  direct  continuity.  Not  only  can  the  ectoderm,  EC,  be  followed  around  from 
the  embryo  over  the  yolk-sac,  but  also  a  layer  of  mesoderm.  The  part  of  the 
yolk-sac  which  carries  the  yolk-grains  is,  as  above  stated,  a  modification  of  the 
entoderm.  There  is  no  amnion. 

The  Umbilical  Cord. 

The  umbilical  cord  may  be  best  defined  as  the  tissues  connecting  the  body 
proper  of  the  embryo  with  the  amnion.  It  accordingly  includes  a  portion  of  the 
body-stalk  and  a  certain  extent  of  the  body-wall  or  somatopleure.  In  early 
stages  we  can  hardly  speak  of  an  umbilical  cord,  because  the  amnion  arises 
close  to  the  embryo _x  (Fig.  69).  As  development  progresses  the  body-stalk 
lengthens  out  (Fig.  $^,  and  the  amnion  arising  from  it  recedes  further  and 
further  from  the  embryo,  this  recession  being  assisted  by  a  growth  of  the  somat- 
opleure which  leads  to  the  formation  of  the  umbilical  cord,  Um,  proper.  By 
this  means  a  tubular  structure  is  produced,  the  cavity  of  the  tube  being  a  pro- 
longation of  the  coelom  of  the  embryo.  During  the  first  development  of  the 


110 


THE  EARLY  DEVELOPMENT  OF  MAMMALS. 


umbilical  cord  the  neck  of  the  yolk-sac,  Vi,  becomes  constricted  and  very  much 
lengthened  out,  forming  the  yolk  or  vitelline  stalk,  Vi.  s.  The  yolk-stalk  springs 
within  the  embryo  from  the  wall  of  the  intestine,  runs  through  the  coelom  of  the 
umbilical  cord,  and  makes  its  exit  beyond  the  amnion,  as  shown  in  the  figure. 
The  yolk-sac  proper  still  occupies  its  original  position  between  the  amnion  and 
chorion.  The  student  should  note  carefully  that  the  umbilical  cord  is  never 
covered  by  the  amnion,  for  it  has  unfortunately  been  often  stated  that  it  is  so 
covered.  An  idea  of  the  relations  can  be  gathered  from  cross-sections  (Fig.  51 ). 
The  coelom,  COB,  is  a  large  cavity  and  contains  the  yolk-stalk,  Y,  with  two 
blood-vessels,  but  with  its  entodermal  cavity  entirely  obliterated.  Above  the 
body-cavity  is  the  duct  of  the  allantois,  All,  lined  by  entodermal  epithelium,  and 
in  its  neighborhood  are  two  arteries  and  a  single  vein.  In  yet  earlier  stages  there 


FIG.  51. — SECTIONS  OF  Two  HUMAN  UMBILICAL  CORDS. 

A,   From    an  embryo    of    21    mm.  ;    B,   from    an    embryo   of  sixty -four  to  sixty-nine    days.     All,    Allantois. 
Ar,   Umbilical  artery.      Cce,  Coelom.     v,  Umbilical  vein.      Y,  Yolk-stalk. 

are  two  veins.  The  outer  surface  of  the  section  is  bounded  by  ectoderm.  The 
further  development  of  the  cord  depends  upon  the  growth  of  the  connective 
tissue  and  blood-vessels,  the  abortion  first  of  the  ccelom,  later  of  the  yolk-stalk, 
and  lastly  of  the  allantoic  duct.  Remnants  of  the  allantoic  epithelium  are, 
however,  often  found  in  the  umbilical  cord  even  at  birth.  There  occurs  also  a 
further  differentiation  of  the  connective  tissue  and  of  the  entoderm. 

The  umbilical  cord  is  characteristic  of  mammals.  It  varies  greatly  in  length . 
In  the  pig  it  is  very  short  and  in  man  it  attains  great  length  and  size,  becoming 
at  full  term  about  55  cm.  in  length,  and  12  mm.  in  thickness.  When  fully  de- 
veloped, it  has  a  whitish  color  and  presents  a  twisted  appearance  somewhat  like 
a  loop.  Its  surface  is  smooth  and  glistening.  The  attachment  of  the  cord  to  the 


THE   UMBILICAL   CORD.  Ill 

embryo  is  known  as  the  umbilicus.     This  attachment  to  the  chorion  is  in  the  pla- 
cental  region. 

The  twisting  of  the  cord  is  well  marked  externally  at  the  time  of  birth  by 
the  spiral  ridges,  within  each  of  which  a  large  blood-vessel  runs.  The  number 
of  spirals  varies  from  3  to  32,  the  turns  beginning  at  the  embryo,  though  usually 
from  left  to  right,  but  sometimes  from  right  to  left.  The  twisting  begins  about 
the  middle  of  the  second  month.  Its  cause  is  unknown,  but  there  is  no  reason  to 
assume  that  it  is  due  to  revolutions  of  the  embryo.  The  cord  is  covered  by  a 
layer  of  epithelium  which  is  continuous  at  the  distal  end  with  the  epithelium  of 
the  amnion,  and  at  the  proximal  end  with  the  epidermis  of  the  embryo.  It  con- 
tains typically  no  capillaries,  and,  except  in  the  immediate  neighborhood  of  the 
embryo,  no  nerve-fibers. 


CHAPTER  III. 
.     THE  HUMAN  EMBRYO. 

Our  knowledge  of  the  early  stages  of  human  development  is  very  imperfect. 
Upon  the  fertilization  and  segmentation  of  the  ovum  in  man  there  are  no  obser- 
vations whatever  at  present.  It  is  not  even  known  exactly  how  long  the  ovum 
requires  for  its  passage  through  the  Fallopian  tubes.  The  earliest  stage  of  which 
we  have  any  comparatively  adequate. account  is  that  represented  by  the  ovum 
described  by  H.  Peters  in  1899.  A  number  of  human  embryos  in  various  early 
stages  after  the  formation  of  the  medullary  canal  and  up  to  the  stage  with  four 
aortic  arches  have  now  been  reported  and  studied ;  some  few  of  them  thoroughly 
and  carefully. 

Calculation  of  the  Age  of  the  Human  Embryo. 

The  age  of  the  embryo  must  be  reckoned  from  the  date  of  the  fertilization  of 
the  ovum,  which  presumably  occurs  in  man  in  the  upper  third  of  the  Fallopian 
tube.  It  may  be  that  ova  become  fertilized  at  various  epochs,  but  fail  to  con- 
tinue their  development  except  when  the  fertilization  occurs  at  the  beginning  of 
a  menstrual  period.  Ovulation  occurs  at  all  periods,  but  most  frequently  about 
the  time  of  menstruation,  which  is  the  expression  of  structural  changes  in  the 
uterus  which  enable  the  ovum  to  implant  itself  in  the  uterine  wall.  Hence  only 
when  fertilization  coincides  with  the  beginning  of  menstruation  can  conception 
follow  with  the  result  that  the  menstrual  flow  is  stopped.  Accordingly,  the  age 
of  the  embryo  is  usually  to  be  reckoned  from  the  date  of  the  beginning  of  the 
first  menstrual  period  which  has  lapsed. 

Experience,  however,  shows  that  sometimes  conception  occurs  without 
stopping  the  menstrual  change  at  the  time,  but  eliminating  only  the  subsequent 
periods,  and  in  such  cases  the  age  must  be  estimated  from  the  beginning  of  the 
last  menstruation.  In  the  two  cases  the  age  of  the  embryo  would  differ  by  a 
month  (twenty-eight  days),  and  this  difference  is  so  great  that  it  obviates  errors 
of  estimate. 

Up  to  the  end  of  the  ninth  week  the  form  and  size  of  the  embryo  exhibit  a 

112 


THE  CLASSIFICATION  OF  THE  EARLY  STAGES.  113 

correlated  development,  so  that  generally  an  embryo  at  a  given  stage  of  develop- 
ment in  form  will  agree  with  its  fellows  in  size;  but  to  this  rule  there  are  not  in- 
frequently exceptions,  and  sometimes  an  embryo  is  found  much  larger  than 
others  at  the  same  stage.  Moreover,  the  variability  of  embryos  is  very  great, 
for  in  specimens  otherwise  alike  we  find  this  or  that  organ  advanced  or  retarded 
in  its  development,  as  compared  with  the  embryo  as  a  whole.  Nevertheless  it 
is  possible  with  the  information  at  command  to  determine  with  tolerable  cer- 
tainty the  age  of  an  embryo  within  two  days  plus  or  minus,  up  to  the  end  of  the 
ninth  week.  For  the  course  of  development  during  the  third  week  we  possess 
as  yet  no  satisfactory  data,  but  embryos  of  full  three  months  are  quite  frequently 
obtained,  and  are  very  characteristic  in  size  and  configuration  (see  page  89). 

The  Classification  of  the  Early  Stages. 

Any  attempt  to  divide  embryos  into  stages  must  necessarily  establish  artifi- 
cial groups,  for  in  nature  there  is  no  demarcation.  Division  into  stages  is  for 
convenience,  and  ought,  therefore,  to  be  made  by  natural  and  obvious  character- 
istics. It  seems  to  me  that  eleven  stages  may  be  conveniently  discriminated, 
as  follows : 

First  Stage. — Segmentation  of  the  Ovum:  The  general  process  is  described 
on  pages  54  to  59.  There  are  no  observations  upon  this  stage  in  man. 

Second  Stage. — Blastodermic  Vesicle:  The  general  development  of  the  blasto- 
dermic  vesicle  in  mammals  is  described  on  page  60.  Its  development  in  man  is 
unknown.  During  this  stage  the  embryonic  shield  is  differentiated.  An  ovum 
of  a  monkey  in  this  stage  is  described  on  page  121;  the  single  human  ovum  known 
is  described  on  page  123. 

Third  Stage. — Primitive  Streak:  No  human  ovum  with  a  primitive  streak 
before  the  formation  of  the  medullary  plate  has  been  observed. 

Fourth  Stage. — The  Medullary  Plate:  In  this  stage  there  are  several  embryos 
known.  In  all  of  them  the  amnion  and  chorion  are  already  differentiated. 
There  is  a  large  extra-embryonic  ccelom.  The  chorionic  vesicle  is  rounded  and 
somewhat  flattened.  In  its  greatest  diameter  it  measures  from  8  to  10  mm.  It 
is  beset  with  short  branching  villi  which  were  found  present  over  the  entire 
surface,  except  in  one  case  described  by  Reichert.  The  general  relations  are 
indicated  in  the  accompanying  diagram  (Fig.  54).  The  chorion  has  a  distinct 
epidermal  and  mesodermal  layer  and  bears  villi.  To  its  inner  surface  is  attached 
the  body-stalk  which  unites  the  embryo  and  chorion.  From  it  springs  the  am- 
nion covering  tjie  embryo,  which  measures  only  i.o  to  1.5  mm.,  and  from  the 
ventral  surface  of  the  embryo  arises  the  yolk-sac,  which  is  of  rounded  form  and 
about  equal  in  diameter  to  the  length  of  the  embryo. 


114 


THE  HUMAN  EMBRYO. 


Fifth  Stage. — The  Medullary  Groove:  The  general  relations  of  the  embryo 
and  its  appendages  are  the  same  as  in  the  previous  stage  (compare  Fig.  66).  In 
the  cases  recorded  the  chorionic  vesicle  varied  greatly  in  size.  It  bore  villi  over 
its  entire  surface,  and  the  villi  were  considerably  branched.  The  embryos  varied 
in  length,  but  measured  about  2.2  mm.  The  medullary  ridges  are  very  charac- 
teristic, rising  high  above  the  yolk-sac  and  enclosing  a  deep  medullary  groove 
between  them.  Of  this  stage  our  knowledge  is  very  imperfect. 

Sixth  Stage. — Medullary  Tube:  In  this  stage  the  medullary  groove  is  partly 
or  wholly  closed  and  the  heart  is  clearly  differentiated.  The  embryo  measures 


FIG.  52. — HUMAN  EMBRYO  AT  THE  BEGINNING  OF  THE  THIRD  WEEK. 
All,  Allantois.     Am,  Amnion.     br,  Branchial  region.     H,  Head.     Hr,  Heart.      Yk,  Yolk-sac. 

from  2.2  to  2.5  mm.  in  length.  The  head  projects  in  front  of  the  yolk.  The 
primitive  segments  are  partly  developed.  In  one  case  seven,  in  another  thirteen, 
were  found  to  have  been  formed.  The  caudal  end  of  the  embryo  also  projects 
beyond  the  yolk,  but  less  than  does  the  head  (compare  Fig.  68).  The  auditory 
invagination  is  probably  not  yet  formed.  There  are  no  gill  clefts  showing  ex- 
ternally. 

Seventh  Stage. — One  Gill  Cleft  Showing  Externally:  Not  known  by  obser- 
vation. 

Eighth  Stage. — Two  Gill  Clefts  Showing  Externally:  Several  embryos  in  this 


THE  CLASSIFICATION  OF  THE  EARLY  STAGES.  115 

stage  have  been  found  and  some  of  them  accurately  studied.  They  usually  have 
a  remarkable  bend  in  the  back  (Fig.  52),  which  imparts  to  the  embryo  a  very 
singular  appearance.  Nothing  similar  to  this  bend  or  dorsal  flexure  has  been 
observed  in  any  other  embryos.  It  has  been  held  by  His  and  others  to  be  a 
normal  condition,  and  not  the  accidental  result  of  a  mechanical  strain  exerted 
by  the  yolk-sac.  If  the  condition  is  normal,  it  must  exist  for  only  a  very  brief 
period,  as  it  is  not  encountered  in  older  or  younger  stages.  We  may  suppose 
if  it  is  normal  that  the  change  from  the  concave  to  the  convex  position  of  the 
embryo,  as  found  in  the  next  stage,  is  very  abrupt.  The  head  of  the  embryo 
(Fig.  52)  shows  the  characteristic  head  bend,  and  the  tail  end  of  the  embryo  is 
also  bent  over  ventralwards.  The  heart  is  large  and  very  protuberant.  It  is 
bent  so  that  we  can  clearly  distinguish  the  auricular,  ventricular,  and  aortic 
limbs.  It  shows  distinctly  its  inner  endothelial  portion  and  outer  mesoderm. 
The  yolk-sac  extends  from  the  heart  backward  to  where  the  body  of  the  embryo 
turns  to  make  the  dorsal  flexure.  Between  the  heart  and  the  head  the  oral 
invagination  has  been  formed,  but  is  still  separated  by  the  oral  plate  from  the 
entodermic  canal.  Above  the  heart  on  either  side  is  an  open  invagination  of  the 
ectoderm,  the  anlage  of  the  so-called  otocyst,  which  in  its  turn  is  the  anlage  of 
the  epithelial  labyrinth  of  the  adult  ear.  In  one  embryo  of  this  stage  there  were 
sriape  found  twenty-nine  primitive  segments. 

Ninth  Stage. — Three  Gill  Clefts  Showing  Externally:  This  is,  on  the  whole, 
the  best  known  of  the  early  stages  of  human  development.  The  embryos  de- 
scribed as  belonging  to  it  vary  from  2.6  to  4.2  mm.  in  length.  In  one  of  them,  in 
which  the  embryo  measured  3.2  mm.,  the  chorionic  vesicle  measured  n  by  14 
mm.,  and  its  supposed  age  was  from  twenty  to  twenty-one  days.  The  general 
of  these  embryos  is  indicated  by  figure  74. 

The  head  is  bent  down  and  the  back  is  very  convex.  In  figure  74  the 
tail  is  rolled  up  and  turned  to  the  left.  Usually,  however,  the  tail  turns  to  the 
right  and  the  head  is  twisted  to  the  left,  so  that  the  long  axis  of  the  body 
describes  a  large  segment  of  a  spiral  revolution;  the  spiral  form  is  marked  in 
embryos  a  little  older. 

Tenth  Stage. — Four  Gill  Clefts  Showing  Externally:  There  are  no  satisfactory 
observations  on  this  stage  in  man. 

Eleventh  Stage. — Appearance  of  the  Limb-buds:  The  embryo  is  much  rolled 
up,  so  that  the  head  and  tail  overlap;  four  slight  protuberances  appear  as  the 
beginnings  of  the  limbs ;  the  cervical  sinus  is  commencing  by  the  invagination  of 
the  posterior  gill  arches. 


116 


THE  HUMAN  EMBRYO. 


Hypothetical  Development  of  the  Blastodermic  Vesicle  in  Primates. 

As  there  exist  no  direct  observations  on  the  earliest  stages  of  man,  we  can 
only  surmise  what  those  stages  may  be.  It  is  evident  that  there  is  a  very  preco- 
cious development  of  the  mesoderm,  of  the  extra-embryonic  ccelom,  of  the 
amnion,  and  of  the  trophoblast,  because  these  four  features  are  found  very 
marked  in  the  earliest  known  stages  alike  of  man,  apes,  and  monkeys.  There  are 
certain  rodents  and  insectivora  in  which  these  same  peculiarities  occur  more  or 
less  emphasized,  in  the  earliest  stages  of  which  we  possess  knowledge.  If  we 
utilize  these  data  as  a  basis,  we  can  reconstruct  the  following  hypothetical  scheme 
of  the  earliest  stages  in  man. 

The  accompanying  diagrams  (Figs.  53  and  54)  represent  three  successive 


EC 


Ent. 


FIG.  53. — Two  DIAGRAMS  TO  ILLUSTRATE  THE  HYPOTHETICAL  EARLY  DEVELOPMENT  OF  PRIMATES 

A»i.c,  Amniotic  cavity.      Coe,  Coelom.     EC,  Ectoderm,  in    B,  bearing  the  anlages  of  villi.     Ent,    Entoderm. 

Mesf ,  Somatic  mesoderm.      Afes//,   Splanchnic    mesoderm.      Tro,  Trophoblast. 

purely  hypothetical  stages  of  the  human  ovum.  They  are  all  conceived  to 
represent  longitudinal  sections.  In  the  first  stage  the  ectoderm,  EC,  forms  a 
moderate  sized  vesicle  and  is  already  thickened.  It  should  probably  be  con- 
ceived as  consisting  of  an  inner  distinctly  cellular  layer  and  an  outer  much 
thicker  trophoblastic  layer  which  is  thickest  over  what  corresponds  to  the  em- 
bryonic region.  This  special  thickening  is  marked  Tro  in  diagram  A.  The 
entoderm,  Ent,  forms  a  small  vesicle  underlying  the  thickened  portion  of  the 
trophoblast.  The  mesoderm,  Mes,  is  well  advanced  in  its  development  and 
already  contains  the  large  extra-embryonic  coelom,  Coe,  and  is  therefore  divided 
into  one  layer  which  surrounds  the  entoderm,  and  a  second  layer  which  underlies 
the  ectoderm.  In  other  words,  the  splanchnopleure  and  somatopleure  are 
already  differentiated.  In  the  next  stage  (Fig.  53,  B)  there  has  been  a  growth, 
the  ovum  has  become  larger,  the  trophoblast  has  increased  in  thickness,  and  in 


THE    ORIGIN    OF   THE   AMNION. 


117 


the  mass  of  thickened  ectoderm  overlying  the  yolk-sac  there  has  appeared  a 
cavity, — the  future  amniotic  cavity, — which  is,  of  course,  entirely  surrounded  by 
ectoderm.  The  portion  of  the  ectoderm  on  the  under  side  of  this  cavity  consists 
of  a  single  layer  of  cells  which  by  assuming  a  cylindrical  form  constitute  the 
thickened  area  which  we  can  identify  as  the  embryonic  shield  (compare  Fig.  18 
and  Fig.  53,  B).  The  solid  mass  of  ectoderm  above  the  amniotic  cavity  is 
later  to  form  a  part  of  the  amnion  and  part  of  the  chorion.  At  the  posterior 
end  of  the  embryo  there  appears  a  considerable  accumulation  of  mesoderm  (Fig. 
54,  b.  s),  which  is  the  anlage  of  the  body-stalk.  Into,  this  the  entoderm  has 
grown  in  the  form  of  a  cylindrical  tubular  prolongation,  the  anlage  of  the  allan- 
tois.  As  a  consequence  of  the  growth  of  the  trophoblast  and  of  the  formation  of 


Ent. 


FIG.  54. — DIAGRAM  OF  AN  EARLY  STAGE  OF  A  PRIMATE  EMBRYO. 

All,  Allantois.     Am,  Amnion.     b.s,  Body-stalk.      Cho,  Chorion.     Emb,  Embryo.     Ent,  Entoderm.      In, 
Entodermal  cavity  of  embryo.      Vi,  Villi  of  chorion.      Yk,  Yolk-sac. 


the  amniotic  cavity,  the  embryo  or  embryonic  shield,  Emb,  together  with  the 
yolk-sac,  Yk,  attached  to  it,  has  been  forced  down  into  the  interior  of  the  cho- 
ri.onic  vesicle.  This  phenomenon  is  very  marked  in  certain  rodents  and  leads  to 
the  so-called  inversion  of  the  germ-layers.  In  the  next  stage  the  amnion  is 
formed.  This  is  accomplished  by  the  penetration  of  the  mesoderm  with  accom- 
panying extensidh  of  the  extra-embryonic  ccelom  into  the  mass  of  the  ectoderm 
overlying  the  amniotic  cavity  (compare  Figs.  53,  B,  and  54)  until  the  condition 
shown  in  figure  54  is  brought  about.  This  is  the  stage  known  by  observation. 


118  THE  HUMAN  EMBRYO. 

The  amnion,  Am,  is  now  completely  separated  from  the  chorion,  Cho,  which  forms 
a  relatively  large  vesicle  and  consists  of  a  thin  layer  of  mesoderm,  and  a  very 
thick  layer  of  ectoderm,  which  has  an  inner  cellular  stratum  and  an  outer  very 
much  thicker  trophoblastic  stratum.  The  trophoblast  is  now  very  much  altered 
by  the  appearance  of  numerous  spaces  or  channels  in  it  which  develop  so  that 
each  of  these  spaces  ends  blindly  toward  the  interior  of  the  chorion,  but  many  of 
them  are  open  upon  the  surface  of  the  trophoblast.  As  the  ovum  at  this  stage  is 
already  embedded  in  the  uterine  mucosa,  the  channels  in  the  trophoblast  can 
receive  maternal  blood,  and  such  is  their  original  function.  The  embryo  and 
yolk-sac,  as  compared  with  the  chorionic  vesicle,  are  very  small  in  size.  The 
body-stalk,  b.  s.,  is  well  developed  and  contains  a  well-marked  allantoic  anlage, 
All,  formed  by  the  entoderm.  The  embryo  contains  as  yet  very  little,  if  any, 
mesoderm.  Probably  no  neurenteric  canal  exists  at  this  stage.  During  the 
transition  of  stage  B  (Fig.  53)  to  stage  C  (Fig.  54),  the  blood-vessels  appear  in 
the  mesoderm  of  the  yolk-sac. 

Relations  of  the  Embryo  to  the  Uterus. 

The  study  of  Peters's  ovum  and  of  early  stages  of  various  primates  leads  us 
to  conceive  that  the  ovum  first  implants  itself  in  the  mucous  membrane  of  the 
uterus.  The  conception,  "implantation,"  is  the  outcome  of  very  recent  re- 
searches. The  essential  idea  we  have  formed  of  implantation  is  that  the  tropho- 
blast of  the  ovum  corrodes  or  digests  the  uterine  tissues  with  which  it  comes  in 
contact,  and  thus  produces  a  cavity  in  which  it  is  lodged  and  where  it  attaches 
itself  intimately  to  the  maternal  tissues.  Owing  to  this  process  the  ovum  is  at 
first  partly  uncovered,  and  this  condition  seems  to  be  permanent  in  monkeys. 
In  man  and  the  apes,  however,  the  uterine  mucosa  grows  over  the  exposed  por- 
tion of  the  ovum,  forming  a  layer  of  maternal  tissue  which  separates  the  ovum 
from  the  cavity  of  the  uterus.  This  layer  is  the  anlage  of  the  decidua  reflexa. 
As  the  ovum  grows,  the  decidua  reflexa  must  also  expand,  and  we  soon  reach  a 
condition  in  which  the  primitive  relations  of  the  parts  can  be  easily  followed. 

When  the  uterus  becomes  pregnant,  the  mucous  membrane  of  the  organ 
undergoes  changes  in  structure,  and  it  is  then  commonly  no  longer  termed  the 
mucosa,  but  the  decidua  or  caduca.  The  decidual  membrane  is  histologically 
characterized  by  modifications  in  the  glands,  the  epithelium  of  which  in  large 
part  degenerates,  by  the  transformation  of  a  large  number  of  the  connective- 
tissue  cells  into  cells  of  large  size,  which,  on  account  of  their  being  so  extremely 
characteristic,  are  called  the  decidual  cells,  and,  finally,  it  is  characterized  by  a 
growth  of  its  blood-vessels. 

The  decidual  membrane  of  the  uterus  is  divided  into  three  regions :  first,  the 


RELATIONS  OF  THE  EMBRYO  TO  THE  UTERUS. 


119 


decidua  serotina,  the  area  (Fig.  55,  s,s)  to  whicfr  the  ovum  is  attached;  second,  the 
decidua  -vera,  comprising  all  the  remaining  portions  of  the  mucosa  forming  part 
of  the  walls  of  the  body  of  the  uterus ;  third,  the  decidua  reftexa,  the  arching  dome 
of  maternal  tissue,  r,r,  which  rises  from  the  walls  of  the  uterus  and  completely 
encapsules  the  ovum.  The  arrangement  of  the  parts  is  illustrated  in  figure  55, 
which  represents  a  median  section  of  a 
uterus  about  five  weeks  pregnant.  The 
whole  uterus  is  considerably  enlarged. 
The  mucous  lining  of  the  uterus  is  very 
greatly  thickened.  The  ovum  is  at- 
tached on  the  dorsal  side  of  the  uterus. 
This  is  the  normal  position.  The  dia- 
grams so  commonly  met  with  which  rep- 
resent the  insertion  of  the  ovum  at  other 
points  should  not  be  accepted  by  the 
student.  The  reflexa  rises  around  the 
ovum,  completely  covering  it  in  so  as 
to  make  a  closed  bag.  The  ovum  itself 
is  a  sac  known  as  the  chorionic  vesicle. 
The  trophoblast  has  now  quite  disap- 
peared, except  so  far  as  it  persists  to 
cover  the  villi.  The  villi  themselves  are 
shaggy,  more  or  less  branched,  and  their 
tips  are  united  either  with  the  surface  of 
the  decidua  serotina  or  with  that  of  the 
decidua  reflexa.  In  the  interior  of  the 
chorion  is  lodged  the  embryo  with  its 
yolk-sac  and  surrounded  by  the  am- 
nion. 

If  the  walls  of  the  uterus  are  cut 
through  and  simply  reflected,  leaving  the 
bag  of  the  decidua  reflexa  intact,  the  ap- 
pearances will  be  found  essentially  as  in 
figure  56.  The  mucosa  is  enormously 
hypertrophied  and  contains  a  great  many 

dilated  irregular  blood-sinuses.  From  the  dorsal  side  of  the  organ  is  suspended 
a  large  closed  bag  or  sac,  the  decidua  reflexa,  D.  ref,  nearly  filling  the  cavity  of  the 
uterus.  The  reflexa  presents  in  the  stage  figured  the  same  general  appearance  as 
the  surface  of  the  uterus.  If  the  reflexa  be  open,  we  come,  of  course,  upon  the 


FIG.  55. — SEMI-DIAGRAMMATIC  OUTLINE  OF  AN 
ANTERO-POSTERIOR  SECTION  OF  A  HUMAN 
UTERUS  CONTAINING  AN  EMBRYO  OF  ABOUT 
FIVE  WEEKS. 

a,  Anterior,  /,  posterior  surface,  g,  Outer  limit  of 
the  decidua  s,s,  Limits  of  the  decidua  serotina. 
ch,  Chorion,  within  which  is  the  embryo  enclosed 
by  the  amnion,  and  attached  to  the  chorion  by 
the  umbilical  cord ;  from  the  cord  hangs  the 
pedunculate  yolk-sac.  r,r,  Decidua  reflexa. — 
(After  Allen  Thompson.') 


120 


THE  HUMAN  EMBRYO. 


villous  chorion  of  the  ovum,  and  find,  as  above  stated,  that  only  the  tips  of  the 
villi  are  united  with  the  surface  of  the  reflexa.  .  In  the  finished  stage  the 
decidua  is  reddish-gray,  spongy  or  pulpy,  soft,  and  moist.  After  the  fourth 
month  it  acquires,  especially  in  the  superficial  layers,  a  duller  brownish  color, 
which  subsequently  becomes  more  marked.  This  coloration  is  due  to  the  deci- 
dual  cells.  During  the  first  two  or  three  months  the  scattered  openings  of  the 
uterine  glands  can  still  be  distinguished  over  the  surface  of  the  serotina  and  vera. 


Muse 


DV. 


1 


FIG.  56. — HUMAN  UTERUS,  ABOUT  FORTY  DAYS  ADVANCED  IN  PREGNANCY. 

Muse,  Muscularis.  Dv,  Decidua  vera.  D.ref,  Decidua  reflexa.  Ov,  Ovary.  Ovd,  Oviduct  (Fallopian  tube). 
Lig,  Round  ligament.  Vg,  Vagina.  The  uterus  has  been  opened  by  cutting  through  the  anterior  walls 
and  reflecting  the  sides. — (After  Coste.) 

The  surfaces  themselves  of  the  vera  and  reflexa,  though  somewhat  irregular,  re- 
main more  or  less  smooth.  The  inner  surface  of  the  reflexa  is  more  irregular  and 
has  protuberant  parts  united  with  the  tips  of  the  future  chorionic  villi.  The 
surface  of  the  decidua  serotina,  on  the  contrary,  becomes  very  irregular  during 
the  progress  of  pregnancy,  forming  little  mounds  which  may  become  so  high  as 
to  resemble  columns,  or  so  broad  as  to  constitute  septa.  In  later  stages  the  septa 


p 


OVUM  OF  A  MONKEY  IN  THE  SECOND  STAGE.  121 

become  very  well  developed,  attaining  a  height  of  from  5  to  15  mm.  They  are 
irregularly  disposed,  but  subdivide  the  placenta  of  later  stages  into  the  so-called 
cotyledons  (compare  page  337). 

The  body-stalk  becomes  converted  into  the  umbilical  cord.  This  cord  runs 
from  the  body  of  the  embryo  to  the  chorion  (Figs.  55  and  72).  It  is  always  con- 
nected with  that  portion  of  the  chorion  which  is  adjacent  to  the  decidua  serotina. 
It  carries  the  arteries  and  veins  from  the  body  of  the  embryo  to  the  chorion. 
From  the  end  of  the  umbilical  cord  the  blood-vessels  branch  out  over  the  chorion 
and  into  the  chorionic  villi.  Thus  the  chorionic  circulation  of  the  embryo  centers 
about  the  chorionic  end  of  the  umbilical  cord,  and  as  this  end  is  in  the  part  of  the 
chorion  overlying  the  decidua  serotina,  we  have  here  established  from  the  very 
start  an  important  factor  in  the  further  differentiation.  From  what  has  been  said 
it  is  evident  that  the  portion  of  the  chorion  underlying  the  decidua  reflexa  is  more 
remote  from  the  center  of  the  embryonic  circulation.  In  the  same  way  we  find 
that  the  decidua  reflexa  is  remote  from  the  blood-supply  in  the  uterus,  and,  as  a 
matter  of  fact,  we  may  observe  that  during  the  second  month  of  pregnancy  the 
blood-vessels,  both  in  the  decidua  reflexa  and  in  the  portion  of  the  chorion  near 
it,  begin  to  disappear  and  ultimately  are  completely  atrophied.  After  this 
atrophy  has  been  accomplished  the  circulation  of  the  chorion  is  restricted  to  that 
portion  overlying  the  decidua  serotina.  When  the  blood-vessels  of  the  chorion 
under  the  decidua  reflexa  abort,  the  villi  of  that  region  also  abort,  so  that  that 
part  of  the  chorion  becomes  smooth,  and  is,  therefore,  called  the  chorion  lave. 
Over  the  serotina  the  villi  continue  to  grow,  hence  that  region  of  the  chorion 
becomes  known  as  the  chorion  frondosum.  The  chorion  frondosum  constitutes 
the  foetal  portion,  the  decidua  serotina  the  maternal  portion,  of  the  permanent 
placenta.  The  maternal  blood  circulates  in  the  intervillous  spaces,  which  are 
bounded  by  foetal  ectoderm.  The  foetal  blood  circulates  in  the  foetal  blood- 
vessels of  the  chorionic  villi.  The  circulatory  channels  of  mother  and  foetus  are 
always  distinct,  and  no  mingling  of  the  maternal  and  foetal  blood  is  possible 
under  normal  conditions. 

Ovum  of  a  Monkey  in  the  Second  Stage. 

This  embryo  was  obtained  from  a  Semnopithecus  nasicus  in  Borneo  by 
Selenka.  The  ovum  represents  the  earliest  stage  of  any  primate  yet  known. 
It  rested  against  the  wall  of  the  uterus  and  was  uncovered,  there  being  no  decidua 
reflexa  developed  in  monkeys.  It  measured  about  2  mm.  in  its  greatest  diam- 
eter. Figure  57  represents  a  section  through  the  ovum  and  adjacent  tissues  of 
the  uterus.  The  chorionic  vesicle  is  very  large,  but  the  embryo,  Sh,  and  yolk- 
sac,  Yk,  are  relatively  very  small.  The  chorion  on  one  side  is  quite  smooth; 


122 


THE  HUMAN  EMBRYO. 


on  the  opposite  side  it  has  developed  numerous  outgrowths,  most  of  which  are 
formed  exclusively  of  the  ectoderm,  but  a  few  contain  an  ingrowth  of  mesoderm 
in  their  interior.  The  ectoderm  on  the  side  toward  the  uterus  has  two  layers,  an 
inner  cellular  layer  with  relatively  small  nuclei,  and  an  outer  syncytial  or  tropho- 
blastic  layer  with  larger  nuclei  of  variable  size.  The  ovum  occupies  a  depression 
on  the  surface  of  the  uterus  from  which  the  uterine  tissues  have  disappeared, 
with  the  result  of  breaking  through  the  walls  of  some  of  the  blood-vessels,  bl.lac, 
so  that  now  the  maternal  blood  may  escape  from  these  vessels  into  the  spaces  left 
between  the  irregular  outgrowths  and  the  embryonic  chorion.  We  must  assume 


Tro. 


Am.c. 


EC. 


Gl.  bl.lac.  Conn.  Gi. 

FIG.  57. — BLASTODERMIC  VESICLE  OF  A  MONKEY  (SEMNOPITHECUS  NASICUS)  ATTACHED  TO  THK  UTERUS  ; 

VERTICAL  SECTION. 

Ain.c,  Amniotic  cavity,  bl.lac,  Blood  lacuna.  Coe,  Extra-embryonic  coelom.  Conn,  Connective  tissue  of  the 
uterus.  EC,  Ectoderm.  Gl,  Gl,  Uterine  glands.  Mes,  Mesoderm  of  embryonic  chorion.  Sh,  Embryonic 
shield.  Tro,  Trophoblast.  Vi,  Mesodermic  core  of  a  chorionic  villus.  Yk,  Yolk-sac. — (After  £.  Selenka.) 

that  the  trophoblast  of  the  embryo  has  actually  dissolved  away  or  digested  the 
tissues  of  the  uterus,  thus  providing  an  attachment  for  the  ovum,  securing  its 
embedding  in  the  wall  of  the  uterus,  and  establishing  an  opportunity  for  the 
maternal  blood  to  flow  into  the  intervillous  spaces.  In  later  stages  of  the  pri- 
mates the  trophoblast  is  very  much  reduced,  and  therefore  fulfils  its  functions 
in  the^ery  earliest  stages  by  establishing  these  primitive  conditions  of  blood- 
supply. 


HUMAN  EMBRYO  IN  THE  SECOND  STAGE. 


123 


A  section  of  the  embryo  on  a  larger  scale  is  shown  in  figure  58.  There 
appears  only  the  embryonic  shield,  Sh,  which  is  remarkable  for  its  small  area 
arid  great  thickness.  The  yolk-sac  is  also  very  small  and  is  lined  by  a  distinct 
layer  of  entoderm,  Ent.  Above  the  embryonic  shield  is  the  amniotic  cavity, 
which  is,  of  course,  bounded  by  ectoderm  which  is  continuous  with  the  ectoderm 
of  the  embryonic  shield.  The  amniotic  cavity  has  a  curious  extension  into  the 
body-stalk,  b.  s,  by  which  the  embryo  is  connected  with  the  chorion.  The  meso- 
derm  is  chiefly  developed  over  the  chorion,  as  shown  in  figure  57.  It  is  very 
slightly  developed  in  the  embryo  (Fig.  58,  mes),  but  forms  a  layer  over  the  yolk- 
sac  and  over  the  amnion,  and  forms  a  considerable  mass  of  tissue  to  constitute 
the  body-stalk,  6.  s. 


A.  mes. 


A.ec 


Am.c. 


Cce. 


Ent. 


FIG.  58. — EMBRYO  OF  THE  PRECEDING  FIGURE  MORE  HIGHLY  MAGNIFIED. 

Am.c,  Amniotic  cavity.  A. ec,  Amniotic  ectoderm.  A. mes,  Amniotic  mesoderm.  b.s,  Body-stalk.  Cos,  Extra- 
embryonic  ccelom.  Ent,  Entoderm.  mes',  Somatic,  mes" ,  splanchnic,  mesoderm.  Sh,  Embryonic 
shield. — (After  E.  Setenka.} 


Human  Embryo  in  the  Second  Stage. 

The  embryo  to  be  described  was  investigated  by  H.  Peters.  It  was  found 
attached  to  the  dorsal  wall  of  a  uterus  almost  completely  embedded  in  the  mu- 
cosa,  but  it  was  not  wholly  covered  thereby,  so  that  there  was  no  decidua  reflexa 
yet  present.  A  blood-clot  was  found  overlying  what  would  have  been  other- 
wise the  exposed  portion  of  the  ovum.  The  trophoblast  formed  an  enormously 
thick  layer  of  very  irregular  outline  and  contained  many  large  spaces  filled  with 
maternal  blood  (Fig.  59).  The  exact  external  diameter  of  the  ovum  could  not, 
therefore,  be  determined.  It  measured,  however,  approximately  2.4  mm.  by 
1.2  mm.  The  internal  diameter  of  the  chorionic  vesicle  was  about  1.6  by  0.8 
mm.  The  trophoblast  was  everywhere  intimately  united  with  the  uterine 


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^mJ^-^Si^  X':-.'-' ;: 

iaSC&\  iiawss:  ^.-l--**?**1  .  •»  ••.  "  J   .• 


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C  «  H 


THE  HUMAN  EMBRYO  IN  THE  SECOND  STAGE.  125 

tissue.  The  embryo,  Sh,  was  represented  by  an  embryonic  shield  consisting  of 
cylinder  cells.  It  is  small  and  lies  on  the  side  of  the  ovum  away  from  the  cavity 
of  the  uterus.  It  rests  upon  the  small  yolk-sac,  Yk,  and  is  overlain  by  the  am- 
niotic  cavity,  Am.  c,  which  is  bounded  everywhere  by  ectoderm — on  one  side,  of 
course,  that  of  the  embryonic  shield ;  on  the  other  the  thin  amniotic  ectoderm 
proper.  The  mesoderm  extends  around  the  ovum,  forming  a  layer  underneath 
the  chorionic  ectoderm  over  the  yolk-sac  and  above  the  amnion.  At  one  point, 
close  to  the  embryo  and  yolk-sac,  it  encloses  a  triangular  space  the  meaning  of 
which  is  not  known.  As  indicated  in  the  figure,  the  mesoderm  was  found  to  have 
shrunken  somewhat,  and  the  appearance  of  the  embryo  and  yolk-sac  also  sug- 
gests a  somewhat  imperfect  preservation,  histologically  speaking,  of  the  tissues. 
As  regards  the  condition  of  the  uterus,  the  following  points  may  be  noted.  In 
the  neighborhood  of  the  ovum  the  decidua  vera  had  acquired  a  thickness  of  about 
8  mm.,  while  on  the  opposite  or  anterior  side  it  was  only  from  5  to  6  mm.  in  diam- 
eter. Only  in  the  immediate  neighborhood  of  the  ovum  could  there  be  seen  any 
differentiation  of  the  mucous  membrane  into  an  upper,  more  compact  layer,  and 
a  deeper,  looser  cavernous  layer.  The  epithelium  of  the  glands  and  the  tissues 
of  the  uterus  were  well  preserved,  except  in  the  immediate  neighborhood  of  the 
ovum.  The  picture  produces  the  impression  that  the  ovum,  in  order  to  secure 
a  place  for  itself,  has  completely  destroyed  the  uterine  tissues  with  which  it  has 
been  in  contact,  thus  implanting  itself  in  the  maternal  tissue.  And  as  a  conse- 
quence of  the  destruction  of  the  maternal  tissues  the  walls  of  some  of  the  blood- 
vessels have  been  broken  through,  and  this  has  allowed  the  blood  to  escape  from 
those  vessels  into  the  lacunae  of  the  trophoblast. 

The  trophoblast.  of  the  ovum  offers  a  very  complex  picture,  owing  chiefly 
to  the  changes  which  it  is  undergoing.  The  changes  seem  here  due  apparently 
to  hypertrophic  degeneration.  The  layer  of  the  chorionic  ectoderm  next  to  the 
mesoderm  retains  more  or  less  evidently  a  cellular  character.  The  remaining 
portions  tend  to  form  a  syncytium  in  which  the  nuclei  become  enlarged  and  the 
cell-boundaries  obliterated,  while  the  protoplasm  of  the  cells  also  changes  in 
character  and  becomes  more  homogeneous  in  texture  and  much  denser.  The 
syncytium  disappears  by  resorption,  and  its  disappearance  causes  the  formation 
of  spaces  in  the  trophoblast.  Many  different  pictures  occur  in  connection  with 
these  processes,  for  in  some  places  the  nuclei  tend  to  gather  in  groups,  in  others 
they  disappear,  in  some  instances  strands  of  degenerative  material  are  left,  while 
nearby  some  of  the  trophoblast  may  retain  its  more  primitive  appearance  and  be 
but  slightly  altered.  Finally,  it  should  be"  noted  that  at  various  points  the 
chorionic  mesoderm  is  growing  out  into  the  trophoblast.  Each  of  these  meso- 
dermic  outgrowths  is  to  be  interpreted  as  the  anlage  of  the  central  portion  of  a 


126 


THE  HUMAN  EMBRYO. 


chorionic  villus,  and  out  of  the  neighboring  chorionic  ectoderm  will  be  differen- 
tiated the  ectodermal  covering  of  the  villus.  It  seems,  from  a  comparison  of 
later  stages,  that  the  trophoblastic  degeneration  never  goes  so  far  as  to  leave  any 
of  the  chorionic  villi  without  an  ectodermal  covering.  But  this  covering  varies 
extremely  in  its  exact  character  as  we  find  it  in  later  stages,  even  in  adjacent 
parts  of  the  same  villus,  for  it  may  be  either  a  single  layer  of  cells  or  a  layer  of 


Am 


Yk. 


Cho. 


Vi. 


FIG.  60. — EMBRYO  OF  A  GIBBON  (HYLOBATES  CONCOLOR)  IN  THE  THIRD  STAGE. 
Am,  Amnion.      Yk,  Yolk-sac.     Cho,  Chorion.      Vi,  Villi. — (A/ter  E.  Selenka. ) 

cells  covered  by  a  thin  coat  of  syncytium  or  merely  a  syncytial  layer  (compare 
page  342).  The  disappearance  of  all  of  the  trophoblast,  except  so  much  as  re- 
mains to  share  in  forming  the  ectodermal  covering  of  the  villi,  produces  the  so- 
called  intervillous  spaces  of  later  stages,  in  which,  as  above  stated,  maternal 
blood  circulates. 


THE  EMBRYO  OF  A    GIBBON  IN  THE  THIRD  STAGE. 


127 


The  Embryo  of  a  Gibbon  in  the  Third  Stage. 

The  embryo  to  be  described  was  obtained  from  a  female  Hylobates  concolor 
in  Borneo  by  Selenka.  It  still  had  traces  of  the  primitive  streak,  the  anterior 
end  of  which  was  an  open  neurenteric  canal.  The  medullary  plate  was  partially 
differentiated  from  the  embryonic  shield.  It  was  thoroughly  studied  by  Selenka, 
and  is  the  best  known  very  early  stage  of  any  primate.  It  is  more  advanced  than 
the  human  embryo  described  by  Peters  (page  123).  The  entire  ovum  is  repre- 
sented in  figure  60.  The  figure  was  reconstructed  from  a  study  of  the  sections. 
It  shows  the  chorionic  membrane  studded  with  villi.  The  diameter  of  the  cho- 
rion  was  about  8.5  mm.  The  number  of  villi  was  about  one  hundred,  of  which 
some  seventy  are  clustered  about  the  region  where  the  embryo  was  found.  The 


Emb.     Am. 


Am.     EC. 


Ent. 


Yk.  Ve.  All. 

FIG.  61. — EMBRYO  OF  A  GIBBON,  SIDE  VIEW  OF 

THE  EMBRYO  OF  FIG.  60. 
Emb,   Embryo.       Am,  Amnion.       neu,   Neurenteric 

canal,      b.s,  Body-stalk.       Yk,  Yolk-sac.       Ve, 

Blood-vessels.        All,    Allantois.  —  (After    E. 

Selenka. ) 


FIG.  62. — TRANSVERSE  SECTION  OF  THE  EMBRYO 
OF  THE  PRECEDING  FIGURE. 

Am,  Amnion.  EC,  Ectoderm.  F,  Dorsal  furrow. 
mes,  Mesoderm.  Ent,  Entoderm.  Ve,  Blood- 
vessel.— (After  E.  Selenka.) 


others  are  scattered  over  the  surface  of  the  membrane.  They  are  considerably 
branched.  Each  one  is  covered  by  ectoderm  which  consists  of  two  layers,  an 
inner  distinctly  cellular,  and  an  outer  one  in  which  the  cell-boundaries  are  indis- 
tinct and  which  is  known,  therefore,  as  a  syncytium  and  represents  the  remains  of 
the  original  trophoblast.  Each  villus  contains  a  core  of  mesodermic  tissue. 
The  chorionic  membrane  is  represented  as  open  in  order  to  show  the  size  and 
position  of  the  yolk-sac,  Yk,  and  of  the  amnion,  Am,  which  encloses  the  embryo 
as  it  rests  upon  the  yolk-sac.  The  embryo  itself  is  not  shown  in  the  illustration. 
Both  the  yolk-sac  and  the  amnion  are,  of  course,  covered  by  a  layer  of  mesoderm. 
The  entire  space  between  these  two  inner  structures  and  the  chorion  corresponds 
to  the  extra-embryonic  coelom,  the  very  precocious  and  enormous  development 


128 


THE  HUMAN  EMBRYO. 


nch. 


of  which  is  a  special  characteristic  of  primates,  including  man,  and  is  not  at 
present  known  to  be  paralleled  by  the  conditions  in  the  early  stages  in  any  other 
mammals. 

A  side  view  of  the  embryo  on  a  larger  scale  is  represented  in  figure  61.  The 
embryo  is  connected  with  the  chorion  by  a  well-marked  body-stalk,  b.  s,  is  cov- 
ered by  the  arching  amnion,  Am,  and  rests  upon  the  yolk-sac,  which  in  compari- 
son to  the  chorionic  sac  seems  very  small.  The  yolk-sac,  Yk,  already  has  de- 
veloped from  it  a  network  of  blood-vessels,  Ve,  which  contain  blood-corpuscles, 
but  have  not  yet  developed  into  .am  embryo  itself.  The  disposition  of  these  ves- 
sels is  best  illustrated  by  the  section  (Fig.  62).  The  yolk-sac  is,  of  course,  lined 

in  its  interior  by  entoderm.  It  has  formed  already 
a  prolongation,  All,  into  the  body-stalk.  This 
prolongation  is  the  anlage  of  the  future  allantois. 
Figure  63  represents  a  surface  view  of  the  same 
embryo,  or  perhaps  one  should  say,  rather,  of 
the  embryonic  shield.  At  the  posterior  end  there 
is  the  short  primitive  streak,  the  anterior  limit  of 
which  is  marked  by  the  opening  of  the  neurenteric 
canal,  neu,  which  passes  obliquely  downward  and 
forward,  as  shown  also  in  figure  61.  From  the 
end  of  the  neurenteric  canal  there  extends  forward 
a  slight  thickening  of  the  entoderm  which  can  be 
recognized  as  the  anlage  of  the  notochord,  nek. 
Figure  62  represents  a  transverse  section  through 
the  region  of  the  notochord.  It  shows  the  amnion , 
Am,  arching  over  the  embryo,  the  thickened  ecto- 
derm of  the  embryonic  shield,  and  the  anlage  of 
the  notochord,  Ch.  The  mesoderm,  mes,  of  the 
embryo  no  longer  extends  across  the  median 
line,  and  is  without  any  ccelom.  At  the  edge 

of  the  embryo  the  mesoderm  splits  and  one  layer  passes  over  on  to  the 
amnion,  the  other  on  to  the  yolk-sac.  In  the  wall  of  the  yolk-sac,  D,  one 
can  easily  distinguish  a  layer  of  the  entoderm,  Ent,  and  also  in  the  mesodermic 
portion  the  young  blood-vessels,  Ve.  Comparison  with  a  section  of  a  some- 
what older  embryo  of  another  gibbon,  Hylobates  Rafflesi,  also  described  by 
Selenka,  will  be  found  instructive.  The  relations  are  here  similar  to  those 
shown  in  the  section  just  described,  although  the  stage  is  somewhat  more 
advanced,  for  we  see  that  the  amniotic  cavity  is  larger,  that  the  formation  of  the 
medullary  groove  has  begun,  that  the  ccelom  is  beginning  to  appear  in  the  em- 


pr.s 


FIG.  63. — SURFACE  VIEW  OF  THE 
EMBRYONIC  AREA  OF  THE  OVUM 
SHOWN  IN  FIG.  61. 

pr.a,  Primitive  axis,  neu,  Neurenteric 
canal,  nch,  Notochord.  pr.s,  Primi- 
tive streak,  b.s,  Body-stalk. 


HUMAN  EMBRYO  IN  THE  FOURTH  STAGE. 


129 


Yk 


Md.  gr.    --' 


bryonic  mesoderm,  and  that  the  blood-vessels  of  the  yolk-sac  have  increased 
greatly  in  size.  In  this  embryo  there  were  traces  of  the  formation  of  three  seg- 
ments a  little  in  front  of  the  neurenteric  canal  which  was  still  present  and  open. 
This  embryo  was  found  to  be  attached  to  the  wall  of  the  uterus  and  to  be  enclosed 
in  a  decidua  reflexa.  In  later  stages  the  decidua  reflexa  of  the  gibbon  unites 
with  the  decidua  vera,  and  is  then  lost  completely  by  resorption.  The  general 
character  of  the  ovum  and  its  relations  to 
the  uterus  justify  us  in  the  belief  that  it 
is  extremely  similar  to  the  human  embryoj 
at  the  same  stage. 

Am. 

Human  Embryo  in  the  Fourth  Stage  with 
the  Medullary  Plate. 

The  general  relations  in  this  stage 
have  been  indicated  by  the  diagram  (Fig. 
54).  A  more  exact  idea  of  the  embry- 
onic structures  may  be  gathered  from 
figure  65,  which  represents  a  median  sec- 
tion of  the  embryo  taken  from  a  wax 
model  reconstructed  from  the  sections. 
The  general  disposition  of  the  parts 
agrees  very  closely  with  the  previous 
stage  as  described  for  primates.  The  em- 
bryo and  yolk-sac  are  very  small  in  com- 
parison with  the  entire  ovum,  and  they 
are  connected  by  means  of  the  body-stalk, 
b.s,  with  the  chorion,  Cho.  The  body- 
stalk  contains  the  entodermal  anlage,  All, 
of  the  allantois.  The  embryo  is  covered 
by  the  amnion,  Am,  which  arises  in  front 
of  the  head  of  the  embryo,  now  becom- 
ing marked  off,  and  runs  above  the  em- 
bryo to  join  the  distal  end  of  the  body- 
stalk.  The  opening  of  the  yolk-sac,  Yk,  is  about  equal  to  the  length  of  the 
embryo.  The  yolk-sac  is,  of  course,  lined  by  entoderm  and  has  a  thick  layer 
of  mesoderm  supplied  already  with  relatively  large  blood-vessels  containing 
blood-corpuscles;  the  vessels  are  developed  chiefly  upon  the  inferior  hemi- 
sphere of  the  yolk-sac.  The  embryo  (Fig.  65)  measured  1.54  mm.  in  length. 
Its  dorsal  surface  is  represented  in  figure  64.  This  surface  is  occupied  by 


Neu.  c. 


Pr.  gr. 


b.  s. 


Cho. 


FIG.  64. — RECONSTRUCTION  OF  A  HUMAN  EM- 
BRYO 1.54  MM.  LONG.  The  amnion  has 
been  opened  to  show  the  dorsal  surface  of 
the  embryo. 

Yk,  Yolk-sac.  Am,  Amnion.  Md.gr,  Med- 
ullary groove.  Neu.c,  Neurenteric  canal. 
Pr.gr,  Primitive  groove.  b.s,  Body-stalk. 
Cho,  Chorion. — (After  Count  Spee.) 


130 


THE  HUMAN  EMBRYO. 


the  very  broad  medullary  plate  of  thickened  ectoderm.  Toward  the  middle 
of  its  length  the  medullary  plate  is  somewhat  narrower  than  elsewhere. 
Along  its  median  line  runs  the  deep,  narrow,  dorsal  groove  which  at  its 
caudal  end  widens  out  and  disappears.  Just  behind  it  is  the  opening  of  the 
relatively  large  neurenteric  canal,  behind  which  again  follows  a  remnant  of 
the  primitive  groove.  A  transverse  section  a  little  in  front  of  the  middle  of 
the  embryo  is  shown  in  figure  26.  The  ectoderm,  ek,  is  very  much  thickened 
to  constitute  the  medullary  plate;  the  narrow  central  longitudinal  furrow, 


Ent. 


FIG.  65. — HUMAN  EMBRYO  OF  1.54  MM.  MEDIAN  SEC- 
TION FROM  A  WAX  MODEL  RECONSTRUCTED  FROM 
SECTIONS. 

All,  Allantois.  Am,  Amnion.  b. s,  Body-stalk.  C/io,  Chor- 
ion.  EC,  Ectoderm.  Ent,  Entoderm.  mts,  Mesoderm. 
Vi,  Chorionic  villus.  Yk,  Cavity  of  yolk-sac. — (After 
Count  Spee.) 


b--' 


FIG.  66. — HUMAN  EMBRYO  OF  1.54  MM. 
Transverse  section  passing  the  neurenteric  canal 
and  yolk-sac,  am,  Amnion.  ek,  Ectoderm. 
ct,  Amniotic  mesoderm.  g,  Meeting-point  of 
somatopleure  and  splanchnopleure.  df, 
Mesoderm  of  yolk-sac,  b,  b,  b,  Blood-ves- 
sels, en,  Entoderm.  n,  Neurenteric  canal. 
d,  Cavity  of  yolk-sac,  e,  Medullary  plate. 
— {After  Count  Spec.} 


/,  mentioned  above  is  very  noticeable.  Outside  of  the  embryo  the  ectoderm  is 
reflected  on  to  the  amnion,  Ct,  over  the  back  of  the  embryo.  The  entoderm  is 
a  thin  layer  of  cells  in  the  center  of  which  the  notochordal  band  can  be  distin- 
guished, ch.  In  sections  near  the  neurenteric  canal  the  notochord  is  better 
marked,  being  there  much  thicker  than  the  remaining  entoderm.  The  meso- 
derm, me,  is  a  distinct  layer,  although,  as  other  sections  show,  it  is  fused  in  the 
median  line  of  the  primitive  streak  behind  the  neurenteric  canal  with  both 


HUMAN  EMBRYO  IN  THE  FIFTH  STAGE. 


131 


Cho. 


b.s. 


Md. 


Am. 


ectoderm  and  entoderm.  Although  the  extra-embryonic  coelom  is  fully  devel- 
oped, that  of  the  embryo  is  present  as  a  small  fissure,  p,  only.  Figure  66  is 
a  section  passing  through  the  neurenteric  canal,  and  shows,  therefore,  the 
amnion,  am,  the  thickened  medullary  plate,  e,  of  the  embryo,  and  the  large 
yolk-sac,  d.  The  yolk-sac  is  formed,  of  course,  of  splanchnopleure.  The  thick- 
ening of  the  mesodermic  layer  in  the  lower  part  of  the  yolk-sac  in  order  to 
allow  space  for  the  developing  blood-vessels,  6,  6,  6,  is  well  shown  in  the  figure. 
Eternod  has  studied  an  embryo  in  this  stage.  He  finds  that  the  heart  is 
already  present  underneath  the  slightly  projecting  head.  From  its  anterior  end 
it  sends  out  two  aortic  branches  which  run  on  either  side  near  the  notochord,  pass 
in  a  gentle  curve  around  the  neuren- 
teric canal,  come  nearer  together  in  the 
region  of  the  primitive  groove,  and 
enter  the  body-stalk,  through  which 
they  run  parallel  to  the  allantois  and 
form  ramifications  in  the  chorion.  He 
finds  also  two  veins  in  the  body-stalk 
which,  when  they  reach  the  embryo, 
unite  to  a  single  median  trunk,  which 
quickly  divides  into  two  vessels  which 
run  in  the  mesoderm  of  the  yolk-sac 
near  the  embryo  proper  until  they 
reach  the  venous  end  of  the  heart, 
into  which  they  open.  They  each  re- 
ceive a  venous  branch  from  the  caudal 
side  of  the  yolk-sac. 


FIG.  67. — HUMAN  EMBRYO  WITH  OPEN  MEDULLARY 

GROOVE. 

Am,  Amnion.     b.s,  Body-stalk.      Cho,  Chorion.     Md, 
Medullary  folds.      Yk,  Yolk-sac.— (After  W.  His.} 


Human  Embryo  in  the  Fifth  Stage  with  Open  Medullary  Groove. 

Although  several  embryos  in  this  stage  have  been  studied,  none  of  them  has 
furnished  very  thorough  information.  The  two  best  studied  were  recorded  by 
His;  one  he  designates  as  "  E"  and  the  other  as  "  SR"  (Fig.  67).  The  chorionic 
vesicle  of  "E"  measured  8.5  X  5.5mm.;  of  "SR,"  9X8  mm.  The  embryo  in 
"E"  measured  (?)  2.1  mm.;  in  "SR,"  2.2mm.  (Fig.  67).  It  will  be  noticed  at 
once  that  the  condition  is  very  similar  to  that  shown  in  figure  65,  but  the  embryo 
is  somewhat  more  advanced.  The  most  important  changes  in  the  embryo  at 
this  stage  are  its  general  growth,  so  that  it  rises  above  the  yolk  and  has  both 
projecting  head  and  projecting  tail.  The  medullary  groove  is  very  deep  and 
extends  the  entire  length  of  the  embryo.  Toward  its  caudal  end  it  probably  has 
an  open  neurenteric  canal.  The  dorsal  outline  of  the  embryo  is  somewhat  con- 


132 


THE  HUMAN  EMBRYO. 


cave.  On  the  under  side  of  the  projecting  head,  between  it  and  the  anterior 
limit  of  the  yolk-sac,  the  anlage  of  the  heart  has  appeared,  and  its  cavity  may  be 
supposed  to  be  in  connection  with  the  blood-vessels  of  the  yolk-sac.  The  devel- 
opment of  segments  has  begun;  how  many  were  present  in  either  of  these  em- 
bryos is  uncertain.  From  the  under  side  of  the  projecting  tail  end  springs  the 
body-stalk,  to  the  distal  end  of  which  the  chorion  is  attached.  The  chorion  is 
completely  covered  by  short  branching  villi.  The  yolk-sac  has  still  a  very  broad 
connection  with  the  embryo,  and  contains  blood-vessels  throughout  its  entire 
extent.  The  space  between  it  and  the  chorion,  the  extra-embryonic  coelom,  is 
very  large. 


Ht 


FIG.  68. — HUMAN  EMBRYO  OF  FROM  THIRTEEN  TO  FOURTEEN  DAYS. 

Am,  Amnion.     S. 7,  Seventh  segment.     Md,  Medullary  groove,     fft,  Heart.      Yk.s,  Yolk-sac.     Al,  Body-stalk. 

— (After  J.  Kollmann^) 

Human  Embryo  in  the  Sixth  Stage  with  Medullary  Canal. 

This  stage,  if  we  define  it  to  include  the  whole  period  from  the  beginning  to 
the  completion  of  the  closure  of  the  medullary  groove  to  form  the  medullary 
canal,  covers  a  considerable  epoch  of  development.  The  best-known  specimen 
of  this  stage  was  described  by  Kollmann.  It  measured  2.2  mm.  in  length  and 
had  the  medullary  groove  open  through  the  anterior  two- thirds  of  its  length,  but 
closed  along  the  caudal  third.  The  embryo  had  thirteen  segments  (Fig.  68). 
The  yolk-sac  was  attached  to  the  embryo  for  a  distance  of  1.5  mm.,  leaving  the 
head  to  project  0.58  mm.  and  the  tail  to  project  0.3  mm.  The  head  is  already 
somewhat  enlarged  and  slightly  bent  over  toward  the  ventral  side.  It  forms  at 
least  one-third  of  the  whole  embryo.  The  dorsal  outline  of  the  embryo  is  con- 
cave in  the  region  where  the  segments  have  developed.  The  caudal  end  is 
slightly  curved  over  and  is  connected  on  its  under  side  with  the  body-stalk,  Al, 
by  which  the  embryo  is  attached  to  the  chorion.  Between  the  yolk-sac,  Yk.  s, 
and  the  head,  the  heart,  Ht,  is  prominent.  By  analogy  with  other  vertebrates 
we  assume  that  the  heart  tube,  when  it  first  appears  in  man,  is  straight  and 


FIG.  69. — HUMAN    OVUM,   SAID   TO    BE   FROM   FIFTEEN   TO   EIGHTEEN    DAYS  OLD.     (Compare  footnote, 

page  134.) 

The  chorion  has  been  opened  and  spread  out  to  show  the  embryo  and  its  adnexa.     A/,  Body-stalk  containing 
the  allantoic  diverticulum.     Am,  Amnion  surrounding  the  embryo.      Vi>  Yolk-sac. 


133 


134 


THE  HUMAN  EMBRYO. 


occupies  a  longitudinal  median  position.     In  this  embryo  it  has  already  become 
a  relatively  large  organ  and  the  tube  itself  is  strongly  bent.     No  anlage  of  the 

eye  or  ear  was  distinguished.  The  amnion  was  a  thin, 
transparent  membrane  enveloping  the  embryo  quite 
closely.  The  closeness  of  the  amnion  to  the  embryo 
was  probably  accidental  (compare  Figs.  69  and  71). 
The  chorion  was  covered  externally  by  branching  villi ; 
its  diameter,  including  the  villi,  was  18  mm. 

Another  embryo,  the  position  of  which  in  the  series 
of  known  stages  has  long  been  a  matter  of  dispute,  I 
feel,  after  renewed  study,  must  be  assigned  to  a  place 
very  close  to  Kollmann's  embryo  just  described.  The 
specimen  in  question  was  figured  by  Coste  in  his 
monumental  "Atlas  of  Embryology."*  The  embryo 
was  enclosed  in  a  villous  chorion  (Fig.  69)  and  was  pro- 
vided with  a  large  vitelline  sac,  Vi,  having  a  very  broad 
connection  with  the  embryo  and  covered  with  a  network 
of  vessels,  in  which  was  a  fluid  not  yet  red.  A  thick 
body- stalk,  Al,  can  be  seen  running  from  the  under  side 
of  the  embryo's  tail  to  the  chorion;  from  the  anterior 
side  of  the  stalk  springs  the  amnion,  Am,  completely 
inclosing  the  embryo.  It  is  important  to  notice  that  in 
this,  as  in  still  older  embryos,  the  disposition  of  the 
amnion  is  essentially  the  same  as  in  the  earliest  stages ; 
the  line  of  attachment  of  the  amnion  is  down  the  sides 
of  the  allantois  and  around  the  embryo  about  on  a  line 
with  the  top  of  the  yolk.  As  regards  the  embryo,  it  is 
drawn  slightly  canted  on  to  its  left  side;  its  back  is 
concave ;  the  head  end  is  thickest ;  behind  and  below  it 
can  be  seen  the  heart,  already  a  bent  tube,  shining 
through;  and  on  the  dorsal  side,  the  light-looking 

oesophagus  is  distinguishable;  in  the  figure  a  wedge-shaped  shadow  intervenes 
between  the  straight  oesophagus  and  the  bent  heart;  the  heart  causes  a  conspicu- 


FIG.  70. — EMBRYO  OF  FIG.  69, 
SEPARATED  FROM  THE 
YOLK-SAC  AND  VIEWED 
FROM  THE  UNDER  SIDE. 

Am,  Amnion.  Ht,  Heart.  Spl, 
Splanchnopleure  extending 
beyond  the  embryo  to  form 
the  yolk-sac.  S,  Noto- 
chord  with  a  row  of  primi- 
tive segments  on  each  side. 
Al,  Body-stalk. 


*  The  greatest  difficulty  comes  from  Coste's  statement  as  to  the  magnification  of  his  drawings,  according  to 
which  the  embryo  must  have  been  about  4.4  mm.  long,  or  nearly  double  the  length  which  we  now  know  to  be 
normal  for  embryos  in  the  stage  in  which  this  one  seems  to  be.  Other  difficulties  arise  because  Coste  has  given 
no  further  description  of  this  embryo  than  that  which  appears  in  the  explanation  of  his  plate.  Neither  that 
explanation  nor  the  figures  themselves  afford  any  information  concerning  the  dorsal  side  of  the  embryo  or  as  to 
whether  it  had  a  partially  open  medullary  groove  or  not.  Coste's  figures  indicate  that  thirteen  or  fourteen  seg- 


HUMAN  EMBRYO  IN  THE  EIGHTH  STAGE.  135 

ous  bulging  of  the  body  between  the  head  and  the  yolk-sac ;  the  caudal  extremity 
is  thick  and  rounded  and  curves  upward.    Figure  70  is  a  ventral  view  of  the  same 
embryo  after  most  of  the  yolk-sac  has  been  cut  off;  its  walls,  Spl  (splanchno- 
pleure),  are  seen  to  pass  over  without  any  break  into  those  of  the  intestinal  cavity. 
In  the  central  line  the  chorda  dorsalis,  s,  can  be  perceived  through  the  translucent 
dorsal  wall  of  the  intestinal  cavity ;  it  is  flanked  on  each  side  by  the  row  of  square 
segments.     Behind,  we  see  the  large  body- 
stalk,  Al,  and  in  front   the  tubular  heart,       ^      ^iil£~%^^ < '^^     v  -'i£%v 
Ht,  with  a  decided  flexure  to  the  right  of     ^^m^ff^y^^^'^       I  /       p 
the  embryo;  the  anterior  end  of  the  heart    cr^  .  f 

makes  an  opposite  bend,  separating  off  a  7 

limb  which  becomes  the  bulbus  aorta.    The      £$\ 
chorion  consists  of   two   membranes,   one 
of   which   forms   the   uninterrupted   inner 
surface   of   the   chorion,    while    the    outer 
membrane    alone    forms    the    hollow    villi 
(Figs.   69  and  201);  hence,  in  looking  at 
the  inside  of  the  chorion,  we  see  numerous    FlG   7,._HUMAN  EMBRYO,  2.15  MM.  LONG. 
round  openings  which  do  not  penetrate  the  —(After  w.  His.) 

inner   membrane.      Fortunately   we   learn 

from  Kolliker,  who  had  an  opportunity  in  1861  to  examine  the  chorion,  that  the 
outer  membrane  was  epithelial,  with  cells  of  the  same  character  as  in  the  epithe- 
lium of  older  vascularized  villi,  and  that  the  inner  layer  consisted  of  developing 
connective  tissue,  and  carried  fine  blood-vessels.  It  thus  appears  that  Coste 
was  the  first  to  observe  the  role  of  the  epithelium  in  the  growth  of  the  villi. 

Human  Embryo  in  the  Seventh  Stage  with  One  Gill  Cleft  Showing  Externally. 

No  human  embryo  with  only  one  gill  cleft  showing  externally  is  known. 

Human  Embryo  in  the  Eighth  Stage  with  Two  Gill  Clefts  Showing  Externally. 

Several  embryos  in  this  stage  have  been  described  and  some  of  them 
studied  anatomically.  Those  which  are  best  preserved  and  which  we  have 
best  reason  to  think  are  normal  present  a  very  singular  appearance,  owing  to 


merits  were  visible  externally.  The  shape  of  the  head,  the  size  and  curvature  of  the  heart,  the  form  of  the  tail, 
and  the  concavity  of  the  dorsal  outline  in  the  segmented  region  of  the  embryo  all  indicate  an  extremely  close 
resemblance  to  Kollmann's  embryo.  As  Coste's  figures  were  all  made  from  fresh  specimens  freehand,  we  shall 
probably  commit  no  error  if  we  assume  that  the  magnification  was  not  correctly  given.  By  making  this  assump- 
tion I  think  the  difficulties  as  to  placing  Coste's  embryo  vanish. 

Coste's  private  collection  was  said  to  be  at  the  College  of  France,  but  upon  search  this  specjmen  could  not 
be  found,  so  that  attempts  to  ascertain  its  actual  length  were  without  result. 


136 


THE  HUMAN  EMBRYO. 


OP, 


the  deep  bend  in  the  segmented  region  of  the  body  so  as  to  constitute  at  the 
dorsal  outline  of  the  embryo  at  that  point  a  U-shaped  curve  (Fig.  71).  This 
bend  is  known  as  the  dorsal  flexure.  Embryos  of  earlier  stages  have  an  in- 
dication of  this  flexure,  as  shown  in  figure  69.  Until  we  have  intermediate 
stages  we  cannot  be  sure  that  the  assumption  which  seems  natural  is  also  cor- 
rect ;  namely,  that  the  deep  dorsal  flexure  of  figure  7 1  is  merely  an  accentua- 
tion of  the  cavity  on  the  dorsal  side  of  the  embryo 
in  earlier  stages.  In  older  embryos  the  dorsal  flex- 
ure is  normally  absent  (compare  Fig.  73  and  the 
following  figures).  It  is  possible  that  the  change 
from  the  concave  to  the  convex  position  is  very 
abrupt,  and  it  is  not  improbable  that  the  time  of  the 
occurrence  of  this  change  is  variable.  The  head  of 
the  embryo  and  the  tail  both  project  far  beyond 
the  yolk-sac,  which,  however,  still  shows  a  broad 
attachment  to  the  embryo.  The  right-angled 
head  bend  is  well  marked  and  the  region  of  the 
fore-brain  projects  downward  so  as  to  leave  a  de- 
pressed area  between  the  head  and  the  heart.  This 
depression  corresponds  to  the  position  of  the  oral 
cavity.  The  heart  is  large,  protuberant,  and 
considerably  bent,  so  that  we  can  distinguish  its 
three  primary  limbs.  From  the  under  side  of  the 
caudal  end  of  the  embryo  springs  the  stout  body- 
stalk  by  which  the  embryo  is  united  with  the  villous 
chorion.  In  another  figure  of  that  embryo  there 
were  twenty-nine  segments  present.  Above  the 
heart  on  the  side  of  the  pharyngeal  region  two  ex- 
ternal depressions  are  visible  corresponding  to  the 
first  two  gill  clefts.  They  are  elongated  in  a  dorso- 
ventral  direction  and  are  narrow.  This  position  of 
the  amnion  is  well  shown  in  figure  71.  It  arises 
from  the  body-stalk  at  the  side  of  the  embryo  along 

the  yolk-sac  and  cardiac  region,  and  extends  around  the  embryo,  but  is  not 
yet  fitted  closely. 

The  anatomy  of  this  stage  is  known  to  us  chiefly  through  the  observations 
of  His  upon  two  embryos  designated  by  him  as  Lg  and  Sch.  i .  Lg  measured  2.15 
mm. ;  Sch.  i ,  2.20  mm.  The  two  embryos  resemble  one  another  closely.  The  fol- 
lowing description  applies  especially  to  Lg.  The  anatomy  can  be  understood  from 


FIG.  72. — RECONSTRUCTION  OF  THE 
ANATOMY  OF  THE  EMBRYO 
SHOWN  IN  FIG.  71. 

Op,  Optic  vesicle,  o.pl,  Oral  plate. 
Ht,  Endothelial  heart.  Li, 
Liver.  Om,  Omphalo-mesaraic 
vein.  Yk,  Yolk-sac.  All, 
Allantoic  diverticulum  formed 
by  the  entoderm.  u.v,  Umbili- 
cal vein.  Ao,  Aorta.  Of,  Oto- 
cyst.— (After  W.  His.) 


HUMAN  EMBRYO   IN  THE  EIGHTH  STAGE. 


137 


the  accompanying  figure  72.  The  medullary  tube  extends  the  entire  length  of 
the  embryo  and  is  the  principal  component  of  the  head.  From  the  region  of  the 
fore-brain  has  been  formed  an  outgrowth  to  constitute  the  optic  vesicle,  Op.  At 
the  side  of  the  hind-brain  and  on  the  dorsal  side  of  the  pharynx  is  situated  the 
anlage  of  the  ear,  Ot,  which  at  this  stage  is  merely  an  open  invagination  of  the 
ectoderm.  The  region  of  the  mid-brain  is  marked  by  the  head  bend,  so  that  the 
axis  of  the  fore-brain  is  approximately  at  right  angles  to  the  axis  of  the  hind- 
brain.  Another  consequence  of  the  head  bend  is  that  the  lower  process  of  the 
head  is  brought  very  close  to  the  pericardial  chamber  enclosing  the  heart,  Ht. 
Between  the  head  and  the  pericardial  sac  is  situated  the  oral  invagination  or 
future  mouth-cavity,  which  is  still  separated  from  the  entodermal  canal  by  the 


FIG.  73. — HUMAN  EMBRYO  OF  2.6  MM.  LENGTH. — (After  W.  His.} 


oral  plate,  0.  pi,  which  consists  merely  of  a  thin  layer  of  cells  belonging  to  the 
ectoderm  and  entoderm  (compare  page  100).  The  pericardial  chamber  is  large; 
in  the  figure  only  the  endothelial  portion  of  the  heart,  Ht,  is  represented.  Around 
this  endothelial  tube  is  a  second  and  more  bulky  one  from  which  arises  the  mus- 
cular wall  of  the  heart.  The  volume  of  the  heart  is,  therefore,  much  greater 
than  indicated  by  the  figure,  hence  the  large  size  of  the  pericardial  chamber. 
On  the  dorsal  side  of  the  heart,  between  it  and  the  hind-brain,  lies  the  entodermal 
canal,  which  is  here  the  anlage  of  the  pharynx.  It  has  two  diverticula  or  gill 
pouches  which  are  not  indicated  in  the  figure.  On  the  side  toward  the  mouth 
the  endothelial  part  is  continued  beyond  the  pericardial  chamber  and  gives 


138 


THE  HUMAN  EMBRYO. 


off  two  vessels  on  each  side,  the  first  and  second  aortic  arches,  which  pass 
around  the  pharynx  and  unite  again  upon  its  dorsal  side,  and  then,  as  the 
aortae,  Ao,  descend  along  the  ventral  side  of  the  nervous  system,  soon 
uniting  in  the  median  line  to  form  the  single  dorsal  aorta  which  runs  along 
nearly  to  the  tail  of  the  embryo,  where  it  forks;  and  its  branches,  passing  one  on 
each  side  of  the  intestinal  canal,  enter  the  body-stalk  and  run  to  the  chorion, 
where  they  branch  out.  Behind  the  pharynx  the  entodermal  canal  merges  into 


P- 


Car. 


FIG.  74.  —  RECONSTRUCTION  OF  THE 
ANATOMY  OF  THE  EMBRYO  OF  2.6 
MM.  IN  FIG.  72. 

op,  Optic  vesicle.  A,  Ventral  aorta.  O»i, 
Omphalo-mesaraic  vein.  Au,  Umbili- 
cal artery.  Ail,  Allantois.  Car, 
Cardinal  vein.  Vh,  Right  umbilical 
vein.  Ao,  Dersal  aorta.  Jg,  Anterior 
cardinal  vein,  ot,  Otocyst. — (After 
W.  His.} 


the  cavity  of  the  yolk-sac,  Yk,  and  then  beyond 
the  yolk-sac  extends  again  into  the  tail  of  the 
embryo,  forming  an  expansion  there  which  is 
known  as  the  bursa.  From  the  under  side  of  the 
bursa  runs  out  the  allantoic  diverticulum,  All, 
which  extends  as  a  narrow  tube  of  entoderrn 
through  the  allantoic  stalk  to  the  level  of  the 
chorion,  where  it  ends  blindly.  The  pericar- 
dial  chamber  on  its  caudal  side  is  bounded  by 
the  septum  .transversum,  in  which  we  find  the 
anlage  of  the  liver,  Li,  already  present,  and 
through  which,  on  either  side,  the  great  vein 
from  the  yolk-sac,  the  omphalo-mesaraic  or 
vitelline  vein,  passes  to  the  heart.  Of  the 
veins  of  the  embryo  only  the  umbilical,  uv,  is 
shown  in  the  figure.  This  vein  gathers  the 
vessels  from  the  chorion,  passes  through  the 
body-stalk,  then  runs  in  the  somatopleure  of 
the  embryo  to  join  the  omphalo-mesaraic  vein 
and  enter  the  heart.  In  the  figure  only  the 
general  course  of  the  vein  is  indicated.  The 
fact  that  it  is  situated  in  the  somatopleure  could 
not  well  be  shown. 


Human  Embryo  in  the  Ninth  Stage  with  Three 
Gill  Clefts  Showing  Externally. 

Our  knowledge  of  this  stage  is  quite  good.  The  described  embryos  vary  in 
length  from  2.6  to  4.2  mm.  The  chorionic  vesicles  are  about  10  mm.  in  diameter, 
varying  according  to  the  size  of  the  embryo.  Figure  73  and  figure  75  represent 
two  embryos  of  this  stage,  the  latter  being  probably  somewhat  more  advanced. 
The  back  of  the  embryo  is  normally  (or  at  least  usually)  convex.  The  head  is 
bent  to  one  side,  usually  to  the  right,  and  the  tail  to  the  other,  the  whole  embryo 


HUMAN  EMBRYO  IN  THE  NINTH  STAGE. 


139 


having  a  slight  spiral  twist.  The  connection  of  the  yolk-sac  with  the  embryo 
has  diminished  in  size,  so  that  it  may  be  said  to  be  connected  by  a  narrower 
process  or  neck  with  the  body  of  the  embryo.  Although  the  head  and  tail  ends 
of  the  embryo  have  become  further  differentiated,  it  should  be  noticed  particu- 
larly that  there  is  now  a  rounded  mass  which  begins  between  the  mouth-cavity 
and  first  gill  cleft  and  extends  ventral  wards  between  the  mouth  and  the  heart, 
forming  a  rounded  protuberance.  This  mass  of  tissue  between  the  mouth  and 
first  gill  cleft  is  known  as  the  mandibular  process, 
because  it  is  the  anlage  of  the  mandibular  region. 
The  heart  has  grown  and  something  of  its  more 
complicated  form  is  indicated  in  the  external 
modeling  of  the  embryo.  The  anlage  of  the 
future  ear  is  now  a  closed  vesicle  or  otocyst. 
From  the  region  over  the  heart,  almost  the  caudal 
extremity,  the  segments  of  the  body  are  distinctly 
marked  externally. 

The  general  anatomy  of  this  stage  will  be 
understood  by  the  aid  of  the  accompanying  fig- 
ures. Figure  76  is  a  reconstruction  from  sections. 
The  position  of  the  notochord,  Ch,  is  indicated  by 
a  line.  The  pharynx  is  large  and  wide.  It  has 
three  lateral  outgrowths  on  each  side,  1,2,3,  the  gill 
pouches.  In  front  and  near  the  cephalic  end  of 
the  notochord  there  is  a  small  median  outgrowth, 
the  anlage  of  the  hypophysis.  Toward  the  neck 
bend  the  pharynx  becomes  narrower  and  passes 
over  into  the  small  entodermal  tube,  from  which 
we  can  detect  the  outgrowth,  Lu,  which  repre- 
sents the  commencing  formation  of  the  lungs. 
This  narrow  tube  leads  to  the  space  above  the 
yolk-sac,  Yk.  s.  Just  where  it  passes  into  the 
yolk-sac  the  entoderm  has  formed  the  rudiment  of 

the  liver,  Li.  Figure  102  gives  a  view  of  the  anterior  wall  of  the  pharynx  of 
another  embryo.  In  front  is  the  large  opening  of  the  mouth,  M,  the  oral  plate 
between  the  mouth-cavity  and  the  entodermal  canal  having  disappeared. 
This  embryo  being  a  little  older,  the  traces  of  the  four  gill  clefts  can  already  be 
seen,  and  there  are  four  entodermal  gill  pouches.  The  aortic  vessels  are  indi- 
cated by  dotted  lines.  The  cardiac  aorta  reaches  the  pharynx  between 
the  bases  of  the  second  and  third  gill  arches,  and  divides  into  two  branches 


FIG.  75. — HUMAN  EMBRYO  4.2  MM. 

Yks,  Yolk-sac.     Am,   Amnion.      All, 

Body-stalk.—  (After  W.  His.} 


140 


THE  HUMAN  EMBRYO. 


on  each  side.  The  anterior  branch  forks  and  runs  through  the  first  and 
second  arches.  The  posterior  branch  forks,  one  fork  going  to  the  third, 
and  the  other,  after  again  forking,  supplies  the  fourth  and  fifth  branchial 
arches.  This  arrangement  of  the  aortic  branches  is  typical.  Between  the 
bases  of  the  first  and  second  arches  is  a  small  protuberance  which  is  the  anlage 
of  the  tongue  and  is  named  by  His  the  tuberculum  impar.  Studies  of  the  sections 
demonstrate  that  the  cavity  of  the  abdominal  region  (splanchnocele)  has  on  each 
side  of  its  dorsal  surface  a  longitudinal  ridge,  the  commencement  of  the  Wolifian 
body.  The  ridge  already  contains  traces  of  the  canals  of  the  Wolffian  body. 


FIG.  76. — OUTLINE  OF  THE  ENTODERMAL  CANAL 
OF  A  HUMAN  EMBRYO  OF  4.2  MM. 

Ify,  Hypophysis.  /,  2,  j,  Lines  marking  the 
position  of  the  pharyngeal  gill  pouches.  Lu, 
Lungs.  Li,  Liver.  Yks,  Yolk-sac.  Al, 
Allantois.  W,  Wolffian  duct.  Ch,  notochord. — 
(After  W.  His.) 


-V 


FIG.  77. — RECONSTRUCTION  OF  THE  ANATOMY  OF 
A  HUMAN  EMBRYO,  3.2  MM.  LONG,  SHOWING 
THE  ANTERIOR  END  VIEWED  FROM  THE  VEN- 
TRAL SIDE. 

Op,  Optic  vesicle.  Ht,  Heart.  Li,  Liver.  V, 
Allantoic  vein.  Au,  Auricle  of  the  heart.  I, 
2,  3,  4,  Aortic  arches. 


Of  especial  interest  is  the  arrangement  of  the  circulatory  apparatus  (Figs.  74  and 
77).  In  the  first  figure  the  arteries  are  shaded  dark;  the  heart  is  an  S-shaped 
tube  which  is  really  double,  consisting  of  an  inner  endothelial  tube  continuous 
with  the  arteries  and  veins  at  either  end  of  the  heart,  and  which  is  confined  to  the 
heart  itself  and  has  nothing  to  do  with  the  blood-vessels  proper.  The  venous 
end  of  the  heart  lies  near  the  yolk-sac.  It  is  convexed  toward  the  head.  The 
arterial  end  of  the  heart  is  convexed  toward  the  tail.  When  viewed  from  the 
ventral  side,  the  venous  process  of  the  heart  (Fig.  77,  Au)  is  seen  on  the  left  and 


HUMAN  EMBRYO  IN  THE  NINTH  STAGE. 


141 


ot 


D.C 


the  arterial  process,  Hi,  is  seen  on  the  right.  The  heart  is  continued  forward  by 
the  large  aorta  (Fig.  74,  A ),  which  gives  off  five  branches  on  each  side  of  the  neck. 
These  branches  again  unite  on  the  dorsal  side  and  run  backward  to  unite  with 
the  fellow-stem,  and  so  form  the  single  median  dorsal  aorta,  Ao,  which  runs  way 
back  and  terminates  in  two  branches,  A  u,  which, 
curving  round,  pass  out  through  the  body-stalk 
and  supply  the  circulation  of  the  chorion.  The 
five  branches  in  the  neck  are  known  as  the 
aortic  arches.  The  column  around  each  branch 
constitutes  the  so-called  branchial  arch.  Each 
branchial  arch  is  further  marked  out  by  the  gill 
cleft  in  front  of  it  and  behind  it,  as  shown  in 
figure  74.  The  reconstruction  of  the  third  em- 
bryo in  the  side  view  (Fig.  78)  affords  further 
information  concerning  the  disposition  of  the 
heart  and  the  large  blood-vessels.  The  veins, 
as  is  there  shown,  are  (i)  the  anterior  cardinals, 
/,  which  are  often  referred  to  as  the  jugular  veins, 
although  they  are  not  identical  with  the  jugulars 
of  the  adult ;  (2)  the  cardinals  (compare  Fig.  74, 
Car),  or  posterior  cardinals  as  they  are  already 
called;  the  posterior  and  anterior  cardinals, 
coming  from  the  caudal  and  cephalic  regions  re- 
spectively, unite  to  form  a  single  transverse 
stem,  the  duct  of  Cuvier,  D.  C.  (the  posterior 
cardinals  receive  their  blood  chiefly  from  the 
Wolffian  bodies,  and  later  undergo  complicated 
metamorphoses) ;  (3)  the  large  umbilical  or 
allantoic  veins,  Al.D,  which  pass  up  from  the 
chorion  through  the  body-stalk  into  the  soma- 
topleure  until  at  the  level  of  the  septum  trans- 
versum,  above  the  liver,  Li,  they  empty  into 
the  duct  of  Cuvier;  (4)  the  omphalo-mesa- 
raic  or  vitelline  veins,  om,  which  come  up  from 

the  yolk-sac  on  either  side  and  meet  the  ducts  of  Cuvier  at  the  venous  end  of 
the  heart.  This  figure  also  shows  the  disposition  of  the  aortic  arches  and  an 
early  stage  of  the  primitive  internal  carotid  artery,  car.  The  muscular,  but 
not  the  endothelial,  heart  is  represented  in  the  reconstruction. 


ot. 


FIG.  78. — RECONSTRUCTION  OF  THE  AN- 
ATOMY OF  THE  HUMAN  EMBRYO  OF 
4.2  MM.  SHOWN  IN  FIG.  74. 
Otocyst.  J,  Jugular  vein.  car, 
Carotid  artery.  /,  First  aortic  arch. 
Au,  Auricle.  Ven,  Ventricle.  Li, 
Liver.  oin,  Omphalo-mesaraic  vein. 
Al,  Allantoic  diverticulum.  Art, 
Allantoic  artery.  Al.  v,  Allantoic  vein. 
Am,  Origin  of  the  amnion.  D.C, 
Duct  of  Cuvier.— (After  W.  His.} 


142 


THE  HUMAN  EMBRYO. 


Human  Embryo  in  the  Tenth  Stage  with  Four  Gill  Clefts  Showing  Externally. 

Although  a  few  embryos  belonging  to  this  stage  have  been  obtained,  they 
have  yielded  almost  no  satisfactory  information.  However,  concerning  em- 
bryos slightly  older,  in  which  the  posterior  gill  arches  have  begun  to  disappear 
owing  to  the  formation  of  the  cervical  sinus,  we  have  quite  accurate  informa- 
tion; hence,  as  we  know  the  next  younger  and  next  older  stages,  we  can  form 
fairly  accurate  conceptions  of  what  must  be  the  condition  during  the  tenth 
stage  concerning  which  we  lack  direct  observations. 


Cerv.s. 


al 


Op 


B.S 


Um.  f. 


EMBRYO   OF   ABOUT   TWENTY- 
X    about    IO    diams. — (After 


FIG.  79. — HUMAN 
THREE  DAYS. 
W.  His.} 

al,  Anterior  limb-bud.  £.S,  Body  stalk.  Op,  Optic 
vesicle.  //,  Posterior  limb  bud.  iv,  Fourth 
ventricle,  i,  Mandibular  process.  2,  Hyoid 
arch,  j,  4,  Third  and  fourth  gill  arches. 


FIG.  80. — HUMAN  EMBRYO  OF  7  MM.     X  8  diams. 

— (After  f.  P.  Mall.} 
a.l,  Anterior  limb  bud.    Cerv.s,  Cervical  sinus.     A/of, 

Mandibular   process.      MX,  Maxillary   process. 

Na,  Nasal  pit.     S,  Eye.     [/m.c,  Umbilical  cord. 

i,  2,   First  and  second  gill  clefts. 


Human  Embryo  in  the  Eleventh  Stage.     Appearance  of  Limb=buds.    Twenty= 
three  Days. 

In  this  stage  the  embryo  is  found  very  much  rolled  up,  so  that  the  head  and 
tail  come  very  close  together  (Fig.  79) .  It  is  further  characterized  by  the  appear- 
ance of  four  protuberances,  two  upon  each  side  of  the  body,  in  which  we  can 
recognize  the  so-called  limb-buds  or  anlages  of  the  extremities.  As  shown  in  the 
figure,  the  dorsal  outline  of  the  embryo  describes  more  than  a  complete  circle. 
The  embryo  has  a  marked  spiral  twist,  the  head  being  bent  to  the  right,  the  tail 
to  the  left.  The  bending  of  the  body  is  especially  marked  at  the  region  of  the 
mid-brain  or  head  bend,  and  at  the  posterior  limit  of  the  hind-brain.  The  anlages 
of  both  pairs  of  limbs  have  appeared,  but  that  of  the  leg  is  smaller  than  that  of 


HUMAN  EMBRYO  OF  TWENTY-SIX  DAYS. 


143 


the  arm.  The  position  of  the  optic  vesicles  can  be  clearly  recognized  by  a  small 
external  protuberance.  The  four  external  gill  clefts  still  show  clearly.  Between 
the  first  gill  cleft  and  the  mouth  lies  the  mandibular  process,  i ;  between  the  first 
cleft  and  the  second  lies  the  hyoid  or  second  branchial  arch.  The  third  and 
fourth  branchial  arches  are  also  distinct,  but  the  fifth  branchial  arch,  or  that  be- 
hind the  fourth  gill  cleft,  no  longer  appears  externally,  but  has  been  turned 
inward,  this  turning-in  marking  the  first  step  toward  the  development  of  the 
cervical  sinus.  The  tail  overlaps  the  pericardial  chamber.  The  body-stalk, 
B.  S.,  conies  off  on  the  left  side  of  the 
embryo,  so  that  when  the  specimen  was 
obtained  it  lay  with  its  left  side  against 
the  chorion,  the  body-stalk  being  quite 
short.  The  specimen  was  derived  from 
a  chorionic  vesicle  measuring  25  X  30 
mm.  The  greatest  length  of  the  embryo 
was  4  mm. 

Human  Embryo  of  Twenty=six  Days. 

The  embryo  figured  (Fig.  80)  was  de- 
scribed by  Mall.  Another  specimen  almost 
identical  has  been  studied  by  H.  Piper. 
At  this  stage  (Fig.  80)  the  embryo  is  flexed 
upon  itself,  forming  almost  a  complete 
circle.  The  limb-buds  have  increased  in 
size.  The  posterior  branchial  arches  are 
in  process  of  disappearance  by  invagi- 
nation  on  the  ectodermal  side  to  form 
the  cervical  sinus.  The  three  branchial 
arches  are  visible  on  the  right  side,  as 
shown  in  the  figure,  but  four  can  still  be 
seen  on  the  left.  The  body-stalk  is  now 

merged  in  the  formation  of  the  true  umbilical  cord  (compare  page  109).  The  head 
is  bent  down  so  as  partially  to  overlap  the  pericardial  region.  On  its  ventral 
side  it  shows  a  large,  shallow  depression,  Na,  which  is  the  anlage  of  the  future 
nasal  cavity.  The  eye  is  marked  by  the  commencing  formation  of  the  lens,  5. 
The  mandibular  process,  Md,  represents  the  lower  boundary  of  the  mouth,  but  it 
has  an  extension  toward  the  high  nasal  pit.  This  extension,  MX,  is  known  as  the 
maxillary  process.  The  first  and  second  gill  clefts,  i  and  2,  can  be  easily  seen, 
and  that  which  might  at  first  be  mistaken  for  the  third,  Cerv.  s,  is  really  the 


FIG.  81. — HUMAN  EMBRYO  7.5  MM.  IN  MAXIMUM 
LENGTH. — (After  W.  His.} 


144 


THE  HUMAN  EMBRYO. 


opening  of  the  cervical  sinus,  in  which  the  gill  clefts  are  partly  buried.  Between 
the  first  and  second  gill  clefts  appears  the  second  or  hyoid  branchial  arch.  Twenty- 
four  segments  are  indicated  externally  on  the  right-hand  side  of  the  embryo 
shown  in  the  figure.  Between  the  head  and  the  anterior  limb  runs  the  large  peri- 
cardial  region.  Between  the  anterior  limb,  a.  I,  and  the  umbilical  'cord,  Um.  c, 
another  protuberance  shows  externally,  due  to  the  development  of  the  liver. 

Human  Embryos  of  Twenty=seven  to  Twenty=eight  Days. 

Embryos  of  this  age  are  characterized  by  the  extreme  development  of  the 

neck  bend  (compare  Fig.  83).  As  is  well  il- 
lustrated in  figure  81,  the  apex  of  the  neck 
bend  forms,  as  it  were,  the  summit  of  the  em- 
bryo, the  greatest  length  of  which  is  from  7  to 
8  mm.  As  it  changes  from  the  twenty-sixth 
day  we  note  especially  the  increased  distinct- 
ness of  the  nasal  pit  and  of  the  still  open  in- 
Y-agination  of  the  ectoderm  to  form  the  lens  of 
the  eye,  and,  still  more,  the  great  deepening  of 
the  cervical  sinus.  Although  the  cervical  sinus 
is  an  ectodermal  structure,  yet  its  formation  is 
due  to  modifications  of  the  gill  arches,  the  third , 
fourth,  and  fifth  of  which  become  pushed  in- 
ward. At  the  stage  we  are  considering,  the 
process  of  invagination,  though  far  advanced, 
is  not  completed.  It  results  in  the  formation 
of  a  deep  fissure  on  each  side  of  the  neck,  the 
general  nature  of  which  may  be  understood 
from  the  accompanying  figure  82.  The  fifth 
arch  is  turned  in  first,  the  fourth  second, 
and  the  third  last.  The  sinus  when  fully 

formed  is  very  narrow,  so  that  the  surfaces  of  the  branchial  arches  come  in 
contact  with  the  opposite  wall  of  the  sinus.  The  ectoderm  on  two  sides  unites 
and  the  sinus  thus  becomes  closed,  and  is  for  a  time  represented  by  an  epithelial 
cord  connected  with  the  epidermis  or  superficial  ectoderm.  So  far  as  known, 
the  cervical  sinus  entirely  disappears,  but  its  abnormal  persistence  may  account 
for  certain  cysts  occurring  pathologically  in  the  neck.  In  all  parts  of  the  em- 
bryo of  twenty-eight  days  there  is  an  obvious  advance  upon  the  conditions 
found  at  twenty-six  days.  Comparison  with  one  another  of  embryos  in  this 
stage  shows  that  there  is  considerable  variation,  especially  as  to  the  nature  and 


I 


FIG.  82.  —  RECONSTRUCTION  OF  THE 
PHARYNGEAL  REGION  OF  A  HUMAN 
EMBRYO  OF  11.5  MM. 

N,  Nasal  pit.  gl,  Processus  globularis. 
hy,  Hypophysis.  Si,  Sinus  cervicalis. 
Lu,  Lung.  Md,  Mandible.  II,  Sec- 
ond branchial  arch.  X  Io  diams. — 
(After  IV.  His.} 


EMBRYO  OF  TWENTY-EIGHT  DAYS. 


145 


degree  of  curvature  of  the  back,  in  consequence  of  which  specimens  differ  in 
general  form  and  in  maximum  length,  though  agreeing  closely  in  structure. 

Embryos  of  the  Second,  Third,  and  Fourth  Months. 

The  following  descriptions  and  figures  illustrate  the  external  form,  no  at- 
tempt being  made  to  describe  the  anatomical  structures. 

Twenty-eight  to  Thirty  Days. — Embryos  of  8  to  10  mm.  Figs.  83  and  84  show 
an  embryo  of  9.0  mm.  There  is  a  general  resemblance  to  the  stage  last  described, 
but  the  following  changes  may  be  especially  noted,  the  first  and  most  conspicu- 
ous being  the  modification  of  the 
limb-buds,  which  have  increased 
greatly.  The  end  of  each  bud  has 
become  somewhat  flattened,  rounded 
in  outline,  and  marked  off  by  a  slight 
constriction  from  the  portion  of  the 
limb-bud  near  the  body.  The  ex- 
panded, flattened  distal  portion  of 
the  limb  is  the  anlage  of  the  hand 
in  the  upper  limb,  of  the  foot  in  the 
lower.  The  position  of  the  limbs  is 
also  characteristic,  the  arm  project- 
ing in  a  line  nearly  parallel  with 
the  axis  of  the  body,  while  the  leg 
stands  out  at  a  wide  angle.  The 
cervical  sinus  has  entirely  disap- 
peared externally  by  closure  of  its 
orifice.  The  nasal  pit  has  con- 
tracted and  deepened  so  •  as  to  have 
lost  its  original  saucer-like  form  and 
more  truly  to  deserve  its  name  of 

pit.  The  large  cavity  of  the  fourth  ventricle  of  the  brain  has  become  more 
conspicuous.  The  umbilical  cord  has  increased  in  length.  From  its  distal  end 
springs  the  amnion,  beyond  which  there  passes  out  from  the  cord  the  narrow 
stalk  of  the  pear-shaped  yolk-sac.  The  cavity  in  the  interior  of  the  umbilical 
cord  is  a  prolongation  of  the  coelom  of  the  embryo,  and  through  this  coelom  the 
yolk-stalk  takes  its  course.  It  bears  blood-vessels  which  ramify  over  the 
surface  of  the  yolk-sac  proper.  Unfortunately  in  the  figure  the  amnion  is 
not  shown. 

.  Thirty-one  to  Thirty-two  Days. — Embryos  of   10  to  12  mm.     As  a  typical 


FIG.  83. — HUMAN  OVUM  WITH  EMBRYO  OF  9.8  MM. 
THE  CHORION  HAS  BEEN  PARTLY  REMOVED  TO 
SHOW  THE  EMBRYO.  X  2  diams. — (Minot  Collec- 
tion, 275.) 


146 


THE  HUMAN  EMBRYO. 


specimen  of  this  stage  we  may  take  the  embryo  designated  by  His  as  Br.  i ,  which 
measured  1 1  mm.  As  compared  with  the  previous  stage  (Fig.  85),  the  back  has 
straightened  out  somewhat,  though  the  lower  end  of  the  body  is  still  rolled  over. 
The  head  has  risen  and  increased  considerably  in  size.  Between  the  end  of 
the  region  of  the  hind-brain  and  the  level  of  the  arm  the  dorsal  outline  has 
become  slightly  concave.  This  concavity  His  designated  the  "  Nackengrube. " 
The  first  gill  cleft,  owing  to  the  completed  closure  of  the  cervical  sinus,  is  the  only 
one  visible  externally.  It  is  the  anlage  of  the  external  auditory  meatus.  It 
is  separated  from  the  mouth  by  a  prominent  mandibular  arch.  On  the  cephalic 
side  of  the  mouth  the  maxillary  process  has  become- more  prominent,  but  the 


FIG.  84. — EMBRYO  OF  THE  PRECEDING  FIGURE.     X  8  diams. 

two  portions  of  the  maxilla  do  not  yet  meet  in  the  median  line.  The  primitive 
segments  are  still  marked  externally.  The  limbs  show  indications  of  their  tri- 
partite division,  the  fore-limb  being  more  advanced  than  the  hind-limb.  The 
division  of  the  digits  of  the  hand  is  just  indicated.  The  abdomen  bulges,  out, 
owing  to  the  growth  of  the  liver.  There  is  a  true  tail,  which  is  now  near  its  maxi- 
mum development.  The  umbilical  cord  has  lengthened  and  shows  the  com- 
mencement of  its  spiral  twisting.  The  amnion  springs  from  the  end  of  the  cord, 
leaving  only  a  short  stretch  of  the  body-stalk  between  the  cord  proper  and  the 
chorion.  The  amnion  envelops  the  embryo  closely.  In  embryos  slightly  older 
than  these  the  changes  in  form  above  mentioned  have  progressed  further.  The 


EMBRYO  OF  THIRTY-ONE  DAYS. 


147 


FIG.  85.— HUMAN   EMBRYO  OK  n  MM.     X  8  diams.— (After  W.  His.} 


FIG.  86.— HUMAN  EMBRYO  OF  ABOUT  14  MM.     X  5  diams. 


148 


THE  HUMAN  EMBRYO. 


body  is  straighter,  the  head  is  larger,  and  has  risen  so  as  to  be  about  at  right 
angles  to  the  body.  The  concavity  (Nackengrube)  below  the  hind-brain  in  the 
outline  of  the  neck  is  more  marked.  The  limbs  are  longer,  the  fingers  more 
distinct.  Where  the  mandibles  meet  in  the  median  line,  the  separation  of  lip 
and  chin  has  begun. 

Thirty-six  Days. — Embryos  of  14  mm.     The  correlation  of  age  and  size  for 


w.b- 


FIG.  87. — HUMAN  EMBRYO  OF  ABOUT  THIRTY-FIVE  DAYS. 

Vit,  Yolk-sac.  V.S,  Vitelline  stalk.  Am,  Amnion.  C'/i,  Chorion.  Vi,  Chorionic  villi.  Al.v,  Allantoic 
vein.  Al.a,  Allantoic  artery.  IV. b,  Wolffian  body,  n,  Main  vein  through  liver.  Lu,  Lung.  Hy, 
Hyoid  arch.  Md,  Mandibular  arch.  MX,  Maxillary  process.  Of,  Olfactory  pit. — (After  Coste. ) 


this  stage  cannot  be  recorded  as  absolute,  but  we  may  say  that  embryos  of  this 
length  are  about  five  weeks  old.  The  body  is  now  nearly  straight  (Figs.  86  and 
87).  The  limbs  project  beyond  the  outline  of  the  body  in  profile  views.  The 
ventral  outline,  owing  to  the  large  size  of  the  heart  and  liver,  is  very  protuberant, 
and  at  this  stage  we  find  that  the  portion  of  the  umbilical  cord  adjoining  the  em- 
bryo is  greatly  enlarged,  owing  to  the  distention  of  its  ccelom,  so  that  a  large 


EMBRYO  OF  THIRTY-EIGHT  DAYS. 


149 


cavity  is  furnished  in  which  there  are  always  found,  as  indicated  in  figure  84, 
several  coils  of  intestine.  This  protrusion  of  a  portion  of  the  intestinal  canal, 
and  sometimes  even  of  a  small  portion  of  the  liver,  into  the  extra-embryonic 
ccelom  of  the  umbilical  cord  is  a  constant  phenomenon.  It  begins  at  a  somewhat 
earlier  stage  and  continues  for  a  considerable  period.  This  curious  condition  has 
been  observed  in  many  different  kinds  of  mammals  in  the  corresponding  stage. 
Later  on,  the  viscera  are  entirely  withdrawn  from  the  umbilical  cord  and  the 
cavity  itself  is  wholly  obliterated.  Figure  87  is  inserted  to  illustrate  the 
relation  of  the  foetal  appendages  to  the  embryo  at  this  stage  and  in  slightly 
younger  embryos.  The  figure  shows  clearly 
that  the  umbilical  cord  is  a  hollow  prolon- 
gation of  the  body-wall  or  somatopleure 
of  the  embryo,  and  that  the  amnion  springs 
from  its  distal  end.  The  yolk-stalk,  V.S,  is 
very  long  and  narrow.  Its  entodermal 
cavity  is  obliterated.  It  is  the  representa- 
tive of  the  original  broad  connection  be- 
tween the  yolk-sac  and  the  entodermal 
cavity  of  the  embryo,  although  it  is  now 
only  a  small  appendage  of  a  loop  of  the 
intestine.  It  bears  the  blood-vessels  which 
run  from  the  embryo  and  ramify  upon  the 
yolk-sac.  On  the  caudal  side  of  the  umbili- 
cal cord  we  find  the  tissue  of  the  original 
body-stalk  in  which  runs  the  allantoic  vein, 
Al.  i>,  and  the  two  allantoic  arteries,  Al.  a, 
which  ramify  upon  the  chorion,  Ch. 

Thirty-eight  Days. — Embryo  of  16  mm. 
in  a  chorionic  vesicle  of  45  by  40  mm.  The 
age  of  this  specimen  (Fig.  88)  is  known  by 

estimate  only.  This  stage  represents  the  transition  of  the  embryo  to  the 
foetus,  because  after  the  fortieth  day  the  form  is  distinctly  human.  The 
head  has  risen  considerably,  and  the  back  has  straightened  still  more,  the 
rectangular  neck  bend  thus  becoming  emphasized.  The  body  has  become 
still  more  protuberant  on  the  ventral  side,  and  in  side  views  the  arms  no 
longer  reach  to  the  outline  of  the  body. 

Forty  Days.— Embryos  of  19  mm.  The  head  has  risen  far  toward  its  de- 
finite position,  with  the  result  of  a  very  rapid  apparent  increase  in  the  length  of 
the  embryo.  The  change  of  position  of  the  head  results  in  bringing  the 


FIG 


— HUMAN  EMBRYO  OF  ABOUT  16  MM. 
X  5  diams.— (After  W.  His.) 


150 


THE  HUMAN  EMBRYO. 


mid-brain  finally  directly  above  the  hind-brain,  the  embryo  being  con- 
ceived as  having  the  body  vertical.  During  the  elevation  of  the  head  the 
concavity  (N ackengrube]  at  the  back  of  the  neck  is  gradually  obliterated.  In 
both  head  and  rump  the  external  modeling,  which  in  earlier  stages  indicated 
more  or  less  the  position  of  the  internal  organs,  has  become  blurred,  and  in  the 
next  stage  is  found  to  have  nearly  or  quite  disappeared.  The  maxillary  pro- 
cesses have  met  and  united  in  the  median  line.  The  anlages  of  the  eyelids  have 
developed.  The  concha  of  the  ear  is  indicated.  The  arm  reaches  beyond  the 
heart;  the  fingers  appear  as  separate  outgrowths. 

Fifty  Days. — Embryo  of  21  mm.  The  author  has  a  fair  specimen  which 
came  into  his  possession  with  no  history  whatever,  but  it  agrees  very  closely 

with  His's  embryo  Ltz,  of  which  he  fixes 
the  probable  age  as  just  over  seven  weeks. 
The  head  is  nearer  its  final  position  than 
in  figure  88,  and  relatively  larger  in  pro- 
portion to  the  body.  In  the  eye,  cornea 
and  conjunctiva  are  clearly  separated ;  the 
face  has  the  foetal  form,  the  nose,  mouth, 
and  chin  being  fully  marked  off.  The 
arms  are  clearly  divided  into  upper  and 
lower  segments;  the  five  digits  are  well 
developed ;  the  hands  rest  over  the  heart 
and  nearly  touch  one  another.  In  the 
specimen  figured  the  outline  of  the  abdo- 
men is  abnormal.  The  leg  shows  the 
tripartite  division ;  the  toes  are  just  begin- 
ning to  be  free,  but  the  hind-limb  is  much 
less  advanced  than  the  fore-limb.  The 
tail  is  still  a  freely  projecting  appendage. 

Fifty-three  Days. — Embryo  of  22  mm.  The  specimen  (Fig.  89)  is  probably 
not  quite  normal,  but,  except  for  the  extreme  and  unusual  curvature  of  the 
back,  it  agrees  closely  with  His's  embryo  Zw,  which  is  figured  by  him  as  a 
normal  embryo  of  presumably  about  seven  and  one-half  weeks.  The  specimen 
was  received  in  1884  with  the  following  history:  "  Menstruation  began  January 
26th.  February  and  March  slight  show  every  few  days.  Abortion  March 
3oth,"  which  is  insufficient  to  determine  the  age.  As  compared  with  the  last 
stage,  there  are  comparatively  few  changes  of  external  form ;  the  most  note- 
worthy are  perhaps  the  increased  development  of  the  legs  and  feet  and  the 
commencing  disappearance  of  the  free  tail.  At  this  time  the  protrusion  of  the 


FIG.  89. — HUMAN   EMBRYO  OF  22  MM. 
diatns. 


X3 


EMBRYO  OF  SIXTY  DAYS. 


151 


coils  of  the  intestine  into  the  coelom  of  the  umbilical  cord  is  about  at  its  maxi- 
mum. 

Sixty  Days. — Embryo  of  28  mm.  The  specimen  figured  resembles  closely 
in  form,  though  it  is  larger,  His's  embryo  Wt,  which  he  has  determined  as  a  nor- 
mal embryo  of  about  eight  and  one-half  weeks.  The  present  specimen  (Fig.  90) 
came  with  no  data.  The  head  is  still  larger  in  proportion  to  the  body  than  in 
figure  89.  The  face  shows  the  two  lines,  which,  as  seen  in  profile,  mark  the  two 


FIG.  90. — HUMAN  EMBRYO  OF  28  MM. 
X  3  diams. 


FIG.  91. — HUMAN  EMBRYO  OF  32  MM.     X  3  diams. 


ridges  which  run  over  the  cheek,  one  alongside  the  nose  to  the  corner  of  the 
mouth,  the  other  from  the  eye;  these  ridges  are  highly  characteristic  of  the  ninth 
week,  and  traces  of  them  not  rarely  persist  in  the  adult  physiognomy.  The  limbs 
have  grown  considerably,  the  hands  being  lifted  toward  the  face ;  at  the  elbow 
there  is  a  considerable  bend;  the  toes  are  all  free  and  the  soles  of  the  feet  are 
turned  toward  one  another.  The  tail  has  disappeared  as  a  free  appendage.  The 
external  genitalia  are  considerably  developed;  the  clitoris-penis  projects  some 
distance. 


152 


THE  HUMAN  EMBRYO. 


Sixty- four  Days. — Embryo  of  32  mm.  The  specimen  (Fig.  91)  came  with 
the  following  history:  "January  4,  1886,  last  flow  began;  March  13,  1886,  abor- 
tion"; between  these  two  dates  are  sixty-eight  days;  but  as  the  flow  took  place, 
conception  probably  occurred  after  menstruation,  therefore  if  we  deduct  four 
days,  making  the  age  sixty-four  days,  we  shall  probably  be  not  far  wrong.  It 
will  be  noticed  that  the  head  has  not  yet  assumed  its  final  angle  with  the  body. 
On  the  other  hand,  the  protuberance  of  the  abdomen  is  much  reduced,  so  that 
the  body  as  a  whole  has  begun  to  have  a  more  slender  form  than  in  earlier  stages. 
In  this  specimen  the  eyelids  have  not  even  begun  to  meet ;  in  another  they  have 
met  (Fig.  92),  except  just  in  the  center,  where  is  still  a  loophole.  This  specimen 
was  brought  with  the  statement  that  it  was  just  sixty  days.  The  endeavor  to 


FIG.    92. — HUMAN   EMHRYO  OF  34  MM. 
VIEW  OF  FACE.     X  3  diams. 


FRONT 


FIG.  93. — HUMAN  EMBRYO  OF  55  MM. 
FIVE  DAYS. 


SEVENTY- 


get  the  exact  data  was  unsuccessful.  The  large  size,  34  mm.,  and  advanced 
development  of  the  embryo  lead  us  to  consider  the  age  given  as  erroneous,  and  to 
believe  the  true  age  to  be  perhaps  sixty-seven  days. 

Seventy-five  Days. — Embryo  of  55  mm.  We  figure  next  (Fig.  93)  a  foetus 
concerning  which  I  possess  no  data.  Comparison  with  embryos  of  two  and  three 
months  leads  me  to  place  it  a  little  under  half-way  between  them.  The  specimen 
has  essentially  the  configuration  of  the  young  child ;  but  the  head  is  very  large 
and  the  body  slender ;  the  position  of  the  limbs  is  typical ;  the  upper  arm  is  bent 
down,  the  forearm  extends  toward  the  chin;  the  knee  is  bent  so  as  to  throw  the 
foot  toward  the  median  line ;  the  soles  of  the  feet  are  placed  obliquely  facing  one 
another;  the  anlages  of  the  nails  can  be  recognized  on  both  the  fingers  and  toes. 


EMBRYO  OF  THREE   MONTHS. 


153 


Embryos  of  the  eleventh  and  twelfth  weeks  are  very  rarely  obtained.  I  have 
never  had  a  normal  one  of  this  period  with  data  to  determine  the  age. 

Three  Months. — Embryos  of  78  to  80  mm.  In  my  experience  there  is  no 
other  age  at  which  abortion  of  normal  embryos  occurs  so  frequently  as  at  three 
months,  and  I  possess  a  number  of  specimens  of  this  age,  which  agree  very 
closely  with  one  another  in  size  and  form.  The  foetus  drawn  in  figure  94  may  be 
taken  to  represent  accurately  the  appearance  of  the  human  embryo  at  three 
months.  The  position  of  the  limbs  is  typical  for  this  age,  but  there  are  slight 

variations,  in  that  the  hands,  one  or 
both,  may  project  more  and  be  higher 
or  lower;  usually  the  right  foot  lies  in 
front  of  the  left,  but  not  always. 
Figure  95  gives  the  front  view  of  face 
of  the  same  embryo  to  show  the  closed 
eyelids,  the  broad  triangular  nose,  the 
thick  lips,  and  the  pointed  chin. 

Three  and  One-half  Months. — 
Embryos  of  108  to  no  mm.  I  have 
several  specimens  which  represent 


FIG.    94. — HUMAN    EMBRYO   OF   78  MM.     THREE 
MONTHS. 


FIG.    95. — FRONT  VIEW  OF  THE   FACE   OF   THE 
EMBRYO  SHOWN  IN  FIG.  94. 


this  age.  I  figure  two  of  them,  one  to  show  the  natural  attitude  (Fig.  96)  in 
utero,ihe  other  to  show  the  natural  attitude  assumed  by  the  embryo  when 
released  from  its  membranes.  The  first  specimen  came  to  me  with  no  history, 
but  as  it  is  certainly  a  little  larger  than  several  other  foetuses  of  about  one 
hundred  and  six  days,  it  is  probably  a  little  older.  The  head  is  bent  forward 
(Fig.  96) ;  the  back  is  curved ;  the  arms  and  legs  are  both  raised  toward  the 
face;  the  right  leg  is  nearly  straight,  so  that  the  toes  are  brought  against 
the  forehead,  while  the  left  leg  is  bent  at  the  knee,  bringing  the  left  foot 


154 


THE  HUMAN  EMBRYO. 


against  the  right  thigh.  In  this  attitude  the  embryo  fills  out  as  perfectly  as  possi- 
ble an  oval  space,  and  fits,  therefore,  the  cavity  of  the  uterus.  The  second  speci- 
men (Fig.  97)  shows  the  attitude  as- 
sumed by  the  embryo  when  free,  and 
proves  that  the  position  in  utero  (Fig. 
96)  is  a  constrained  one.  This  foetus 
was  received  November  30,  1883.  The 
delivery  took  place  on  the  morning  of 
that  day,  and  the  last  menstruation 
had  ceased  one  hundred  and  six  days 
previously;  the  remarkably  fresh  con- 
dition of  the  foetus  indicated  that  it 


FIG.  96. — HUMAN  EMBRYO  OF  120  MM.     (?  ONE 
HUNDRED  AND  TEN  DAYS.) 


FIG.  97. — HUMAN  EMBRYO  OF  n8MM.     ONE 
HUNDRED  AND  Six  DAYS. 


had  been  dead  only  a  very  short  time,  so  that  we  cannot  be  far  wrong  in  put- 
ting its  exact  age  at  one  hundred  and  six  days. 


FIG.  98.— HUMAN  EMBRYO  OF  155  MM.     ONE  HUNDRED  AND  TWENTY-THREE  DAYS. 

155  * 


156  THE  HUMAN  EMBRYO. 

Four  Months. — Embryo  155  mm.  The  foetus  shown  in  figure  98  came  in  a 
very  fresh  condition,  January  2,  1887,  with  the  statement:  "Conception  said 
to  have  taken  place  September  i,  1886;  foetus  came  away  January  2,  about 
noon."  The  embryo. is  typical  in  size  and  development  for  four  months,  except 
that  the  crown  is  higher  than  usual,  and  the  antero-posterior  diameter  of  the 
head  somewhat  below  the  average. 

The  natural  attitude  in  utero  is  similar  to  that  of  figure  96 ;  the  attitude 
shown  is  that  assumed  by  the  foetus  when  released  from  the  membranes. 


CHAPTER   IV. 
STUDY   OF   PIG   EMBRYOS. 

Methods  of  Obtaining  Embryos. 

The  pig  is  recommended  for  embryological  study  because  specimens  of  the 
embryos  in  sufficiently  early  stages  din  be  obtained  at  the  larger  packing  estab- 
lishments in  considerable  numbers  and  with  little  trouble  or  expense.  When 
this  material  is  not  obtainable,  rabbit  embryos  may  be  substituted,  as  these 
animals  are  easily  kept  and  breed  freely  (compare  page  305).  Owing  to  the  enor- 
mous precocious  development  of  the  chorionic  vesicle  in  pigs,  it  produces  an  en- 
largement of  the  uterus  which  is  usually  sufficient,  by  the  time  the  embryo  has 
attained  a  length  of  6  mm.,  to  be  observable  to  the  untrained  eye.  It  is,  there- 
fore, only  necessary  to  ask  the  man  who  removes  the  viscera  from  the  pigs  to  lay 
aside  for  examination  all  of  the  uteri  which  appear  distended.  These  should  not 
be  turned  about  violently,  but  handled  carefully  and  should  be  opened  immedi- 
ately. As  soon  as  the  ovum  is  exposed  it  will  probably  be  ruptured,  and  there 
will  occur  a  free  outflow  of  opalescent  fluid,  amniotic  and  allantoic.  With  the 
aid  of  fine  forceps  and  a  horn  spoon  the  embryo  may  be  lifted  up — and  it  should 
on  no  account  be  directly  touched— and  transferred  to  a  dish  containing  Miiller's 
fluid,  in  which  the  specimen  should  remain  for  five  or  ten  minutes.  It  is  then 
transferred  with  the  help  of  the  horn  spoon  to  Zenker's  fluid.  Metal  instruments 
cannot  be  used  on  account  of  the  corrosive  sublimate  in  the  Zenker  solution.  In 
one  or  two  hours  the  embryo  should  be  transferred  to  fresh  Zenker  solution  and 
left  therein  a  varying  length  of  time,  according  to  the  size  of  the  specimen.  In 
general  it  may  be  said  that  for— 

Pigs  of    6  to    9  mm., 12  hours. 

"     "  12     "          24     " 

"     "  15    "          36     " 

"       "     20  tO  25      "  48       " 

It  is  undesirable  to  leave  any  specimen  in  the  Zenker  solution  more  than 
forty-eight  hours.  The  Miiller's  fluid  is  used  for  cleaning  the  specimen.  It 

157 


158  STUDY  OF  PIG  EMBRYOS. 

causes  a  granular,  non-adherent  coagulum  to  form  from  the  foetal  fluids.  If  the 
specimen  is  put  directly  into  Zenker's  fluid,  a  fibrous  coagulum  is  formed  which 
often  adheres  closely  to  the  embryo  so  as  to  obscure  its  shape.  Such  a  fibrous 
coagulum  cannot  be  removed  without  injuring  the  embryo.  After  having  re- 
mained a  proper  length  of  time  in  the  Zenker  solution,  specimens  are  further 
washed  for  twenty-four  hours  in  running  water,  and  then  treated  with  alcohol 
and  iodine  in  the  usual  manner. 

The  Making  of  Serial  Sections. 

Specimens  should  be  colored  with  alum  cochineal  in  toto,  then  imbedded  in 
paraffin  and  cut  into  serial  sections  according  to  the  directions  given  in 
Chapter  VIII. 

Selection  of  the  Planes  of  Section  and  the  Stages  for  Practical  Study. 

It  is  customary  to  distinguish  three  fundamental  planes — the  transverse,  the 
sagittal,  and  the  frontal.  It  is  impossible  to  so  define  these  planes  that  the  defini- 
tion shall  be  exact  for  all  stages.  But  in  general  it  may  be  said,  the  reference 
being  had  to  the  entire  embryo,  that  the  transverse  plane  is  one  which  will  be  at 
right  angles  to  the  notochord  and  medullary  tube  at  the  level  of  the  heart ;  that 
the  frontal  plane  will  be  one  at  right  angles  to  this,  passing  symmetrically  through 
the  limbs  of  the  embryo;  and,  finally,  that  the  sagittal  plane  is  one  parallel  to 
the  median  plane  of  the  body.  As  in  younger  embryos  the  form  is  very  asym- 
metrical, both  the  head  and  caudal  end  of  the  embryo  being  twisted  to  one  side, 
the  planes  which  would  be  true  for  the  body  of  the  embryo  in  the  region  of  the 
heart -would  not  be  true  elsewhere.  For  the  practical  use  of  the  student,  there- 
fore, in  these  younger  stages  it  is  better  to  determine  the  direction  of  the  plane 
by  the  floor  of  the  fourth  ventricle,  so  that  by  "  transverse"  will  be  understood 
a  plane  of  section  which  cuts  the  head  of  the  embryo  symmetrically,  no  matter 
how  it  may  cut  the  body,  and  which  runs  parallel  to  the  floor  of  the  fourth  ven- 
tricle (medulla  oblongata).  The  frontal  plane  should  be  perpendicular  to  this 
and  also  cut  the  head  of  the  embryo  symmetrically.  The  sagittal  plane  in  these 
cases  is  also  that  of  the  head  and  not  of  the  body.  Such  planes  are  recom- 
mended because  in  the  study  of  the  sections  more  is  gained  by  having  the  planes 
readily  understood  in  the  region  of  the  head  than  in  the  region  of  the  body.  In 
later  stages,  when  the  body  has  become  straighter,  the  difference  in  planes  for  the 
head  and  the  body  may  be  practically  left  out  of  consideration,  except  that  for 
the  heads  of  older  pigs  when  they  are  cut  alone, — as  on  account  of  the  size  of 
the  body  is  often  desirable, — the  frontal  plane  is  chosen  so  as  to  run  at  right 
angles  to  the  plane  of  the  palate  and  symmetrically  through  the  embryo.  Sec- 


THE  STUDY  OF  THE  EXTERNAL  FORM.  159 

tions  through  the  head  at  right  angles  to  this  may  be  designated  as  horizontal.* 
Students  will  find  that  it  is  very  much  easier  to  study  transverse  and  frontal  sec- 
tions when  they  are  symmetrical.  No  pains,  therefore,  should  be  spared  to 
orient  the  embryo  properly  in  the  microtome  before  the  sections  are  cut. 

Selection  of  the  Stages. — The  most  profitable  stage  to  study  is  that  of  an 
embryo  of  from  9  to  13  mm.  in  length.  Each  student  should  have  three  speci- 
mens of  this  stage,  and  it  is  advantageous  that  the  specimens  given  each  student 
be  approximately  of  the  same  size.  The  embryos  should  be  first  studied  care- 
fully as  to  their  external  form  and  then  cut  into  serial  sections  in  the  transverse, 
sagittal,  and  frontal  planes.  Of  these,  the  transverse  series  should  form  the 
principal  basis  of  study,  and  the  other  series  should  be  used  principally  to  clear 
up  the  student's  conception  of  the  relation  of  parts.  Embryo  pigs  of  the  size 
specified  have  the  typical  class  characteristics  of  mammalian  embryos,  and  may 
readily  be  distinguished  from  the  embryos  of  any  other  class  of  vertebrates.  The 
differentiation  of  the  anlages  of  all  the  important  organs  is  accomplished,  so  that 
these  anlages  can  be  identified  with  certainty  and  their  genetic  relations  to  the 
adult  structures  can  be  clearly  grasped  by  the  student.  At  the  same  time, 
although  the  anatomical  differentiation  is  well  advanced,  the  histological  differ- 
entiation has  made  very  little  progress,  so  that  the  stages  in  question  are  par- 
ticularly instructive  to  beginners.  The  anatomy  of  the  pig  at  this  stage  is, 
therefore,  readily  understood  by  the  student  who  knows  the  general  anatomy  of 
the  adult.  Older  embryos  are  more  complicated  and  yield  such  long  series  of 
sections  that  the  beginner  is  apt  to  be  discouraged.  Younger  embryos,  owing 
to  their  spiral  twisting,  are  exceedingly  difficult  for  students  to  understand  when 
sectioned.  After  having  thoroughly  mastered  the  structure  of  the  pig  embryo 
of  from  9  to  13  mm.,  the  student  may  advantageously  extend  his  study  of  em- 
bryos to  other  sizes.  If,  as  is  done  in  this  work,  the  principal  study  is  made  with 
embryos  of  12  mm.,  the  student  may  proceed  to  make  sections  of  other  stages  as 
follows : 

Pig  embryo  of    9  mm. ,  transverse  and  sagittal  series. 

"         "         "     6     "       transverse  series. 

"         "         "17     "       transverse  series. 

"         "         "  20    "        transverse  and  sagittal  series. 

(Of  the  head  alone,  the  frontal  series.) 
Pig  embryo  of  24  mm.,  of  the  head  alone,  frontal  series. 

The  Study  of  the  External  Form. 

The  student  should  make  a  careful  and  thorough  study  of  the  external  form 

*The  system  of  planes  here  described  is  that  adopted  for  the  Harvard  Embryological  Collection,  and  has 
been  found  convenient  in  practice. 


160 


STUDY  OF  PIG  EMBRYOS. 


of  every  embryo,  and  make,  with  the  aid  of  the  camera  lucida,  an  exact  drawing 
of  every  embryo  before  he  cuts  it  into  sections.  He  will  soon  learn  that  such  a 
drawing  is  almost  indispensable  for  the  interpretation  of  the  sections. 

Under  the  following  paragraphs,  embryos  of  12,  15,  and  20  mm.  are  figured 
and  described  from  specimens  which  have  been  hardened  in  Zenker's  fluid  and 
preserved  in  alcohol.  The  description  of  these  stages  will  be  sufficient  to  enable 
the  student  to  understand  any  of  the  embryos  he  is  required  to  study. 

Ven.      Md.        Au. 


C.S. 


P.L. 


FIG.  99. — PIG  EMBRYO  OF  10  MM. 

A.L,  Anterior  limb.  An,  Auditory,  or  first  gill  cleft.  C.S,  Cervical  sinus.  MJ,  Mandibular  process. 
M.L,  Milk-line.  MX,  Maxillary  process.  N,  Nasal  pit.  Of,  Eye.  P.L,  Posterior  limb.  Seg,  Mus- 
cular segment.  Um,  Umbilical  cord.  Ven,  Floor  of  fourth  ventricle  (medulla  oblongata).  X  8  diams. 


Pig  Embryo  of  10mm.  (Fig.  99). — The  head,  which  is  very  large,  in  compari- 
son with  the  body,  forms  as  a  whole  nearly  a  right  angle  with  the  back,  so  that 
the  dorsal  outline  of  the  head  forms  a  distinct  though  rounded  angle  with  that  of 
the  back;  this  angle  marks  the  position  of  the  neck  bend,  and  corresponds  to  the 
junction  of  the  brain  with  the  spinal  cord.  The  neck  bend  is  one  of  the  most 
marked  and  distinctive  characteristics  of  the  mammalian  embryo,  being  much 


EXTERNAL  FORM  OF  EMBRYO  OF  10  MM.  161 

less  developed  in  birds  and  reptiles,  and  being  absent  in  amphibians  and  fishes. 
It  is  probably  closely  correlated  with  the  cramping  of  the  ventral  cervical  region, 
which  leads  to  the  formation  of  the  cervical  sinus,  C.S,  and  to  the  disappearance 
of  the'  second  to  fourth  gill  clefts.  The  neck  bend  is  so  great  that  the  mandibular 
and  nasa'  regions  of  the  embryo  are  closely  appressed  to  the  cardiac  region  of  the 
body  proper.  The  cephalic  region  has  a  second  flexure,  the  head  bend  proper, 
which  marks  the  site  of  the  mid -brain,  and  in  the  figure  appears  as  a  rounded 
angle  obliquely  above  the  eye.  From  the  mid-brain  one  axis,  horizontal  in  the 
figure,  extends  backward  through  the  region  of  the  fourth  ventricle,  or  hind- 
brain,  V en,  to  the  neck  bend,  while  the  other  axis  extends  vertically  downward 
to  the  region  of  the  fore-brain,  which  is  marked  by  a  rounded  protuberance  in  the 
outline  of  the  head.  The  dorsal  outline  of  the  body  proper  forms  a  long  sweeping 
curve,  ending  in  the  tail;  this  dorsal  curvature  is  another  characteristic  of  the 
amniote  embryo,  the  back  in  the  embryo  of  fish-like  forms  being  relatively 
straight.  It  is  thus  brought  about  that  the  dorsal  side  of  the  embryo  is  two  or 
three  times  as  long  as  the  ventral.  From  the  ventral  side  springs  the  large  um- 
bilical cord,  the  connection  of  which  with  the  body  occupies  practically  the  entire 
length  of  the  ventral  median  line  of  the  abdominal  region  proper.  Above  the 
umbilical  cord  the  protuberant  outline  of  the  cardiac  region  passes  below  the 
nasal,  N,  and  mandibular,  Md,  regions  toward  the  cervical  sinus,  C.S.  The 
long  tapering  tail  extends  near  the  umbilical  cord. 

The  surface  nWH^mg  of  the  embryo  offers  important  features.  Beginning 
with  the  head,  we  obserw  first  the  shallow  depression,  constituting  the  nasal  pit, 
N.  The  eye,  Op,  is  entirely  without  lids;  the  lens  appears  in  the  center  and  is 
surrounded  by  the  outlines  of  the  optic  vesicle.  The  small  size  of  the  eye  is  a 
characteristic  of  the  mammalian  embryo ;  by  which  it  differs  from  all  saurop- 
sida-n  forms,  but  in  certain  other  mammals  the  embryonic  eye  is  slightly  larger 
than  in  the  pig.  Below  the  eye  is  the  maxillary  process,  MX,  which  is  destined 
to  form  the  greater  part  of  the  upper  jaw ;  the  anterior  boundary  of  the  maxillary 
process  is  marked  by  a  shallow  depression,  the  lachrymal  groove,  which  runs 
from  the  angle  of  the  eye,  Op,  to  the  nasal  pit,  N.  The  mandibular  process, 
M";,  out  of  which  the  lower  jaw  is  developed,  is  bounded  in  front  by  a  groove 
separating  it  from  the  maxillary  process,  and  behind  by  a  second  groove,  AM, 
thr  anlage  of  the  future  meatus  auditorius  externus.  This  groove  marks  the 
boundary  between  the  mandibular  process  and  the  first  or  hyoid  branchial  arch, 
and  is  itself  the  ectodermal  member  of  the  first  gill  cleft.  The  fourth  ventricle, 
V en,  or  cavity  of  the  hind-brain,  having  very  thin  walls  for  its  roof,  can  be  read- 
ily distinguished.  The  thickened  floor  of  the  fourth  ventricle  is  the  anlage  of 
the  medulla  oblongata.  The  cervical  sinus,  C.S,  is  an  area  of  invagination, 


162  STUDY  OF  PIG  EMBRYOS. 

presenting  at  this  stage  a  triangular  outline;  within  the  sinus  are  found  the 
external  or  ectodermal  terminations  of  the  second  and  third  gill  clefts.  The  ter- 
ritory of  the  mandibular  process  and  cervical  sinus  corresponds  to  the  pharyngeal 
region.  It  is  the  site  of  some  of  the  most  important,  interesting,  and  compli- 
cated developments  by  which  the  embryonic  is  changed  into  the  adult  anatomy. 
The  body  of  the  embryo  shows  the  position  and  number  of  the  segments, 
Seg,  by  the  external  modeling.  Both  limbs  are  well  advanced,  the  anterior, 
A.L,  more  so  than  the  posterior.  From  the  base  of  the  front  to  the  base 
of  the  hind  limb  extends  the  milk-line,  M.L,  curving  so  as  to  be  nearly  parallel  to 
the  dorsal  outline  of  the  body.  Along  this  line  the  mammary  glands  are  ulti- 
mately developed.  Extending  across  the  body  are  several  shadowy  lines,  shim- 
mering through  the  translucent  body-walls.  One  marks  the  position  of  the 
embryonic  diaphragm;  it  extends  from  the  upper  edge  of  the  anterior  limb  ob- 
liquely downward  toward  the  edge  of  the  umbilical  cord.  Another,  which 
extends  in  a  nearly  straight  line  from  limb  to  limb,  marks  the  ventral  edge  of  the 
large  Wolffian  body  or  mesonephros,  the  dorsal  limit  of  which  is  approximate!}' 
indicated  by  the  milk-line,  M.L.  The  outlines  of  the  smaller  left  dorsal  lobe  of 
the  liver  are  distinct,  and  mark  out  a  pointed  area  immediately  below  the  fore- 
limb,  A.L. 

Pig  Embryo  of  12  mm.     General  Anatomy. 

Anatomical  Reconstruction  from  the  Sections.* — The  four  figures  herewith 
presented  are  based  upon  the  same  series  of  transverse  sections  (No.  5)  from 
which  figures  113  to  121  were  drawn.  The  reconstructions  have  been  made  by 
the  method  described  in  Chapter  VIII.  The  actual  drawings,  especially  as  re- 
gards their  shading  to  indicate  the  modeling  of  the  surfaces,  have  been  partly 
made  from  a  wax  model  of  the  brain  and  a  wax  model  of  the  cavity  ot  the 
pharynx  of  the  same  embryo.  The  umbilical  cord  of  the  embryo  used  having 
been  damaged,  the  loop  of  the  intestine  in  the  umbilical  cord  has  been  added  by 
a  reconstruction  from  another  series  (No.  518)  of  an  embryo  of  the  same  size. 

Reconstructions  are  of  the  greatest  assistance  in  the  study  of  sections,  and 
much  facilitate  the  identification  of  all  the  parts.  Students  using  this  book 
should,  while  examining  their  sections,  constantly  refer  to  the  reconstructions. 
It  is  unnecessary  to  give  elaborate  descriptions  of  each  of  them,  since  the  expla- 
nations of  the  lettering  of  the  figures  will  suffice  for  the  identification  of  all  the 
parts  shown.  Certain  brief  explanations  as  to  each  of  the  figures  are,  however, 


*The  four  reconstructions  were  made  by  Dr.  F.  T.  Lewis,  to  whom  the  whole  credit  for  them  belongs. 
The  two  wax  models  referred  to  were  made  by  Dr.  John  L.  Bremer.  The  author  hopes  that  both  of  these  investi- 
gators will  publish  their  results  in  full  elsewhere. 


FIG.  TOO. 


GENERAL  ANATOMY  OF  EMBRYO  OF  12  MM. 


163 


bas. 


FIG.   101. 


164 


STUDY  OF  PIG  EMBRYOS. 


desirable.  They  illustrate  the  general  topographical  distribution  of  the  organs. 
The  great  volume  of  the  central  nervous  system  as  compared  with  the  remaining 
parts  is  very  striking.  Of  the  other  organs,  the  three  which  are  most  conspicu- 
ous by  their  size  are  the  heart,  liver,  and  Wolffian  bodies.  Another  striking 
peculiarity  of  the  embryo  is  the  great  diameter  of  the  blood-vessels,  and  espe- 
cially of  the  veins,  which  are  of  relatively  enormous  diameter,  being  proportion- 
ately much  larger  than  in  the  adult.  In  marked  contrast  to  this  is  the  small 
diameter  of  the  cavity  of  the  trachea  and  lungs  and  of  the  entire  intestinal  canal. 
Figure  101  represents  in  the  main  a  median  section  of  the  embryo  together 


xso  Oe         Coe. 

FIG.    102. — ANTERIOR  WALL  OF  THE  PHARYNX  OF  A 

HUMAN  EMBRYO  OF  3.2  MM. 

1-5,  Gill  arches;  the  arches  are  separated  from  one  another 
by  the  entodermal  and  the  corresponding  ectodermal 
gill  pouches ;  the  aortic  arches  are  drawn  in  dotted 
lines  and  arise  from  the  median  cardiac  aorta.  M, 
Mouth.  Oe, Oesophagus.  Coe,  Coslom.  X  5°  diams. — 
(After  IV.  His.) 


FIG.  103. — AORTIC  SYSTEM  OF  His's  EMBRYO, 
Bl,  4.25  MM. 

I-V,  Aortic  arches.  Uk,  Mandible.  Si/,  Thy- 
roid gland.  K,  Main  aorta.  P,  Pulmonary 
artery.  Lg,  Lung.  Oe,  CEsophagus.  X  3° 
diams.—  (After  W.  His.} 


with  the  organs  of  the  right  side,  but  with  two  exceptions,  first,  the  floor  of  the 
pharynx  is  represented  as  if  cut  through  considerably  to  the  left  of  the  median 
plane;  second,  the  heart  cut  to  the  left  of  the  median  plane.  The  brain  and 
spinal  cord  are  drawn  as  if  opened  to  show  the  modeling  of  the  inner  surface  of 
the  medullary  tube.  The  pharynx  is  so  drawn  as  to  indicate  something  of  the 
modeling  of  its  floor  surface.  The  opening  of  the  veins  into  the  heart  and  of  the 
auricle  into  the  ventricle,  and  the  interventricular  orifices,  are  shown.  Of  the 
digestive  canal  only  the  entoderm  is  represented,  so  that  the  figure  displays  the 
entodermal  walls  of  the  oesophagus,  stomach,  and  intestine,  and  shows  the  two 


FIG.  104. 


. 


Rec 


W.D 


FIG.   105 


FIG.  105. — PIG  EMBRYO  OF  12.0  MM.    RECONSTRUCTION  FROM  THE  TRANSVERSE  SECTIONS,  SERIES  5. 

The  figure  illustrates  chiefly  the  veins  of  the  right  side,  and  shows  the  right  auricle  and  ventricle  of  the  heart. 

A,  The  right  umbilical   artery,  only  a  small  portion  being  drawn  as  it  curves  around  the  Wolffian  duct, 

W.D.     a,  Tip  of  aortic  septum,  which  divides  the  aortic  limb  of  the  heart  into  the  pulmonary  aorta,  P, 

and  main  aorta,  Ao  ;  by  a  growth  of  the  cardiac  tissue,  a,  b,  and  c  of  the  figure  become  joined,  shutting  oft 

the  space  around   the  base  of  the  aorta ;  this  space  communicates  by  the  interventricular  foramen  with  the 

left  ventricle,  and  serves  as  the  permanent  or  adult  channel  of  communication  between  the  true  aorta,  Ao, 

the  left  ventricle.     All,  Allantois.     Ao,   Aortic  division  of  the  aortic  limb  of  the  heart.     Au,  Right 

auricle,      b.  See' "a."     c,  See  "a."     card',  Superior  part  of  the  cardinal  vein  (the  anlage  of  the  azygos). 

i'f ,  Inferior  part  of  the  posterior  cardinal  vein.      cl.  i,  Opening  of  the  first  gill  pouch   into  the  pharynx, 

'ix  being  indicated  by  dotted  shading,      cl.2,  Opening  of  the  second  gill  pouch  into  the  pharynx. 

,  cl-4,  Entodermal  portions  of  the  third  and  fourth  gill  clefts,     c.oni,  Dotted  outline    of  the  omental 

( -sser  peritoneal  cavity,     d,  Left  duct  of  Cuvier.     D.  C,  Right  duct  of  Cuvier,  the  main  venous  trunk 

-ring  the  heart  i   >m  the'  right  side.      D.  V,   Ductus  venosus    Arantii.      F.  W,   Foramen   of  Winslow, 

v/n  in  black.     F.ppt  Foramen,  drawn  in  black,  between  the  pleural  and  peritoneal  cavities.     Gen,  Genital 

tubercle,  represented  as  somewhat  displaced  from  the  median  line,  which  it  really  occupies.      G.R,  Genital 

ge.      /lit? .  ./•'-    '•  Jugular  or  anterior  cardinal  vein.      Li,  Liver,      l.s,  Anlage  of  the  lateral  venous  sinus. 

.   of  the   inferior  maxilla  or  mandible.      P,  Pulmonary  division   of  the   aortic   limb   of  the  heart. 

ty.     PI,  Dotted  outline  of  pleural  cavity.     Rec,  Rectum.     Sc,  Subcardinal  vein,  which 

from  the  cardinal  and  on  the  right  side  of  the  body  forms  part  of  the  vena  cava  inferior.     Scl, 

ian  vein,      s.'s,  Anlage  of  the  superior  longitudinal  venous  sinus,  which  is  formed  by  the  union  of  the 

.,  from  the  sides  to  make  a  single  vessel  between  the  cerebral  hemispheres.      Ur,  Ureter.      Um.d, 

ical  vein,     v,  Valve  of  the  sinus  venosus.      Ven,  Right  ventricle  of  the  heart.      V.H.C,  Vena 

i   jj,in^.»  -,  ...munis      v  op,  Ophthalmic  vein.      V.P,  Portal  vein.      W.b,   Wolffian  body.      W.D,  Wolffian 

d'ict.      x,   Anastomosis  between  the  cardinal  venous  systems  of  the  right  and  left  sides.      X  14  diams. 

:/wn  by  Dr.  F.   T.  Lewis.) 


FIG.  104. 


GENERAL  ANATOMY  OF  EMBRYO  OF  12  MM. 


165 


Gen 


cU 


Ct.2. 


cl.3 


.cU- 


-W.b. 


AIL- 


FIG.  105 


166  STUDY  OF  PIG  EMBRYOS. 

pancreatic  anlages.  Similarly  only  the  entodermal  portion  of  the  trachea  and 
lungs  is  included,  and  the  same  is  true  of  the  caudal  end  of  the  Wolffian  duct 
and  of  its  outgrowth,  which  forms  the  anlage  of  the  kidney.  The  same  is  further 
true  of  the  gall-bladder,  of  which  only  the  epithelial  portion  is  represented.  In 
this  figure  the  arterial  system  is  fully  displayed.  The  pulmonary  artery  and  the 
aortic  trunk  are  completely  separated.  A  small  artery  from  the  pulmonary 
arch  to  the  lungs  is  included,  and  the  figure  shows  the  entire  system  of  branches 
from  the  main  aorta. 

The  development  of  the  aortic  arches  requires  a  few  words  of  special  ex- 
planation. The  disposition  of  the  vessels  is  perhaps  more  clearly  shown  in  figure 
1 02.  In  an  earlier  stage  the  heart  lies  immediately  under  the  pharynx  and  gives 
off  from  its  aortic  end  the  short  aortic  trunk  which  runs  upward  toward  the  floor 
of  the  pharynx.  From  the  a6rtic  trunk  spring  five  pairs  of  vessels,  known  as  the 
aortic  arches  (Fig.  102).  Ten  vessels  are  symmetrically  placed,  five  on  each  side 
of  the  pharynx.  They  pass  around  the  pharynx  dorsalwards.  The  five  vessels 
of  each  side  very  soon  become  united  by  a  dorsal  longitudinal  trunk,  the  descend- 
ing aorta,  which  passes  down  through  the  cervical  region  of  the  embryo  and 
meets  its  fellow  at  about  the  level  of  the  diaphragm,  unites  with  it,  and  thus 
forms  the  single  median  dorsal  aorta.  The  first  arch  very  soon  disappears  (Fig. 
103).  Its  prolongations  with  the  vessels  on  the  ventral  side  run  forward  to  the 
lower  jaw  and  give  rise  to  the  external  carotid.  The  vessel  on  the  dorsal  side  also 
persists  and  gives  rise  to  the  internal  carotid.  Presently  the  second  arch  also 
disappears,  and  both  carotids  are,  as  it  were,  thereby  lengthened.  This  is  the  con- 
dition which  we  find  in  our  embryo  of  12  mm.  (Fig.  101).  The  third,  fourth,  and 
fifth  arches  are  still  present.  From  the  base  of  the  third  arch  runs  forward  the 
external  carotid,  and  from  the  summit  of  the  third  arch  runs  forward  the  internal 
carotid.  The  dorsal  ends  of  the  third  and  fourth  arches  are  still  connected,  but 
this  connection,  instead  of  being  a  large  aortic  vessel,  as  in  earlier  stages,  has  now 
contracted  and  almost  disappeared,  and  will  soon  be  lost  altogether,  so  that  in 
the  adult  there  will  be  no  connection  between  the  dorsal  ends  of  the  third  and 
fourth  arches.  The  fifth  arch  is  still  connected  with  the  dorsal  end  of  the  fourth. 
It  gives  off  the  small  pulmonary  artery  to  the  lungs.  On  the  side  toward  the 
heart  the  relations  of  the  arches  are  also  changed.  The  main  aortic  vessel  which 
springs  from  the  heart  is,  in  the  12  mm.  pig,  divided  into  two  vessels — the  pul- 
monary aorta  on  the  ventral  side  a^J  the  true  aorta  in  a  more  dorsal  position. 
The  division  has  so  taken  place  that  the  third  and  fourth  arches  are  connected 
only  with  the  true  aorta,  while  the  fifth  arch  is  connected  only  with  the  pulmo- 
nary aorta.  The  part  of  the  fifth  arch  on  the  left  side  between  the  origin  of  the 
pulmonary  artery  proper  and  the  main  descending  aorta  offers  at  this  stage  an 


\ 

' 
\ 


FIG.  106 


GENERAL  ANATOMY  OF  EMBRYO   OF  12  MM. 


167 


Cbl. 


M.b 


Ve*.  IV 


Md.ob 


FIG.  107. — PIG  EMBRYO  OF  12.0  MM.  RECONSTRUCTION  FROM  THE  TRANSVERSE  SECTIONS,  SERIES  5. 
To  show  especially  the  cephalic  nerves,  c.i,  c.2,  c.j,  Cervical  nerves.  Cbl,  Cerebellum,  com,  Ganglionic 
commissure  connecting  with  the  jugular  ganglion,  j.  Dien,  Diencephalon.  ex,  External  branch  of  the 
spinal  accessory  nerve.  F,  Froriep's  ganglion,  which  in  man  completely  disappears.  G.j,  Gasseriau 
ganglion.  H,  Cerebral  hemisphere.  ;',  Jugular  ganglion.  L,  Lens.  M.b,  Mid-brain.  Mdb,  Mandibular 
process.  Md.ob,  Medulla  oblongata.  MX,  Maxillary  process,  n,  Ganglion  nodosum.  Nay  Nasal  pit. 
Op,  Optic  cup.  Of,  Otocyst.  rec.l,  Recurrent  laryngeal  nerve.  Ven.IV,  Roof  of  fourth  ventricle,  j, 
Oculomotor  nerve.  4.,  Trochlear  nerve.  5,  Trigeminal  nerve.  6,  Abducens  nerve.  7,  Facial  nerve. 
8,  Vestibular  ganglion.  9,  Petrosal  ganglion,  ro,  Vagus  nerve.  //,  Spinal  accessory  nerve.  12,  Hypo- 
glossal  nerve,  j"  op  marks  the  ophthalmic  division  of  the  fifth  nerve ;  8  is  on  the  vestibular  ganglion, 
immediately  below  which  lies  the  cochlear  ganglion  ;  the  seventh  nerve  has  two  branches,  of  which  the 
anterior  becomes  the  chorda  tympani ;  of  the  two  branches  of  the  ninth  nerve,  the  anterior  becomes  the 
tympanic,  and  the  posterior  the  lingual  nerve  of  the  adult.  X  20  diams. — (Drawn  by  Dr.  F.  T.  Lewis.} 


168  STUDY  OF  PIG  EMBRYOS. 

open  communication  between  the  pathways  of  the  pulmonary  and  of  the  main 
body  circulation.  This  dorsal  half  of  the  fifth  aortic  arch  is  known  as  the  ductus 
arteriosus.  It  remains  throughout  the  foetal  period  as  an  open  channel,  so  that 
the  blood  from  the  right  ventricle  flows  in  part  to  the  lungs,  in  part  into  the  dorsal 
aorta.  The  lumen  of  the  ductus  arteriosus  disappears  in  man  soon  after  birth. 
As  an  anomaly  it  occasionally  persists  throughout  life, 'involving  serious  modifi- 
cations of  the  normal  circulation.  The  dorsal  part  of  the  fifth  aortic  arch  of  the 
right  side  has  a  different  history,  for  it  aborts  early  in  embryonic  life,  and  there 
also  occurs  an  abortion  of  the  entire  descending  aorta  from  the  end  of  the  fourth 
arch  on  the  right  side  downward  to  the  level  of  the  diaphragm.  When  this 
abortion  has  taken  place,  the  entire  aortic  stream  flows  from  the  heart  to  the  left 
side  of  the  embryo.  The  aortic  branches  on  the  right  side  appear  as  follows  in 
the  adult:  The  main  stem,  from  which  the  five  arches  originally  sprang,  is  the 
arteria  innominata,  which  gives  off  a  stem  from  which  spring  the  two  carotids. 
Next  a  vessel  which  represents  the  persistent  third  right  arch,  which  no  longer  has 
any  direct  communication  with  the  aorta,  but  at  its  end  gives  off  the  subclavian 
and  vertebral  arteries.  The  vessel  which  corresponds  to  the  right  third  arch  is 
usually  described  as  a  portion  of  the  stem  of  the  subclavian  artery  in  the  adult. 

Figure  105  is  in  many  respects  similar  to  figure  101 ,  and  is  intended  to  show 
chiefly  the  disposition  of  the  veins.  There  are  also  included  in  this  fig'ure  the 
Wolffian  body  and  its  duct.  The  pharynx  and  the  heart  are  supposed  to  have 
been  cut  through,  well  to  the  right  of  the  median  plane.  This  makes  it  possible 
to  indicate  in  the  figure  the  origin  of  the  pulmonary  aorta  and  of  the  true  aorta. 
The  following  are  the  most  important  veins:  the  umbilical,  which  passes  around 
the  umbilical  opening  and  enters  the  liver;  the  portal  vein,  which  receives  the 
blood  from  the  abdominal  viscera,  and  also  delivers  it  to  the  liver.  In  this  speci- 
men there  is  quite  a  wide  and  free  connection  within  the  liver  between  the  portal 
and  umbilical  veins.  In  other  embryos  of  this  size  such  a  connection  does  not 
always  exist.  The  large  vena  cava  inferior  is  on  the  right  side  of  the  embryo, 
and  also  shows  the  liver,  which  receives  blood  directly  from  the  Wolffian  bodies 
and  from  the  cardinal  veins.  From  the  upper  side  of  the  liver  the  hepatic  vein 
goes  directly  to  the  heart,  uniting  with  the  ducts  of  Cuvier.  These  ducts  receive 
the  jugular  veins  from  the  head  and  the  cardinal  veins  from  the  body.  The  car- 
dinal veins  are  now  very  much  changed.  In  earlier  stages  they  extended  from 
the  duct  of  Cuvier  almost  the  entire  length  of  the  embryo.  Of  this  great  vessel 
there  now  remains  connected  with  the  duct  of  Cuvier  only  a  comparatively  short 
vessel. 

Figure  107  shows  the  disposition  of  the  cephalic  and  upper  cervical  nerves 
and  also  the  position  of  the  nasal  cavity,  the  eye,  and  the  otocyst. 


FIG.  108. 


GENERAL  ANATOMY  OF  EMBRYO  OF  12  MM.  169 


Oe. 


FIG.  109. — PIG  EMBRYO  OF  12.0  MM.  RECONSTRUCTION  FROM  THE  TRANSVERSE  SECTIONS,  SERIES  5. 
The  embryo  has  been  drawn  as  if  transparent,  to  show  the  form  of  its  pharynx  and  the  relations  of  the  pharyn- 
geal  gill  pouches  to  the  grooves  on  the  outer  surface  "of  the  embryo.  Cbl,  Cerebellum.  C.S,  Cervical  sinus. 
Dien,  Diencephalon.  H,  Cerebral  hemisphere.  Hy,  Hypophysis,  which  arises  as  an  evagination  from  the 
oral  cavity.  Inf,  Infundibular  gland,  which  arises  as  a  median  evagination  from  the  floor  of  the  fore-brain. 
l.gr,  Lachrymal  groove,  m,  Maxillary  process.  M.b,  Mid-brain.  Mdb,  Mandibular  process.  Md.ob, 
Medulla  oblongata.  na.ex,  External  nasal  opening,  ni,  A  thin  epithelial  plate  separating  the  nasal  pit 
from  the  oral  cavity  ;  by  the  rupture  of  this  plate  the  internal  nares  is  formed  later.  Of,  CEsophagus.  Of, 
Eye.  Of,  Otocyst.  par,  Part  of  the  thymus,  arising  from  the  epithelium  of  the  fourth  gill  cleft.  Sp.c , 
Spinal  cord.  T/i,  Median  thyroid  gland,  tkym,  Thymus.  Tra,  Trachea.  7,  Entodermal  pouch  of  the 
first  branchial  cleft,  the  anlage  of  the  Eustachian  tube  and  tympanum.  2,  Entodermal  pouch  of  the  second 
gill  cleft ;  it  actually  opens  to  the  exterior,  j,  Entodermal  pouch  of  the  third  branchial  cleft.  4,  Ento- 
dermal pouch  of  the  fourth  gill  cleft,  the  lower  fork  of  which  is  the  anlage  of  the  lateral  thyroid.  /,  Ecto- 
dermal  groove  of  the  first  branchial  cleft ;  it  is  the  anlage  of  the  meatus  auditorius  externus.  X  2O  diams. 
— (Drawn  by  Dr.  F.  T.  Lewis.} 


170 


STUDY  OF  PIG  EMBRYOS. 


Figure  109  gives  an  outline  of  the  head  and  combines  an  indication  of  the 
extenal  modeling  of  the  gill  arches  with  a  representation  of  the  shape  of  the 
pharynx. 

Pig    Embryo   of    15    mm.  —  As   compared   with   figure    99,    the    present 
•  embryo  (Fig.  no)  has  not   only  grown  in  all   its  dimensions,  but  has   also 


FIG.  no. — PIG  EMBRYO  OF  15  MM.     X  8  diams. 


changed  in  form.  Unlike  the  embryo  proper,  the  umbilical  cord  has 
grown  very  little.  We  notice  at  once  that  the  outline  of  the  back  is 
less  curved  than  before,  the  ventral  side  of  the  body  has  acquired  a  con- 
vex outline,  and  at  the  head  it  has  become  considerably  larger,  both  abso- 


EXTERNAL  FORM  OF  EMBRYO  OF  20  MM.  171 

lately  and  relatively,  to  the  body  of  the  embryo,  and  has,  moreover,  risen  so  that 
the  neck  bend  is  diminished.  The  limbs  are  beginning  to  show  the  differentia- 
tion of  the  feet.  Examined  more  carefully,  the  embryo  offers  the  following 
details :  the  eye,  which  is  characteristically  small,  has  become  almond-shaped, 
and  the  circular  lens  can  be  seen  in  the  midst  of  it.  In  the  embryos  of  rodents, 
carnivores,  and  primates  the  eye  is  larger  relatively  than  in  the  pig.  By  the 
growth  of  the  facial  region  the  development  of  the  snout  has  been  initiated  and 
the  opening  of  the  nasal  pit  now  appears  as  the  external  nares  toward  the  end  of 
the  short  snout.  The  lower  jaw  is  clearly  differentiated  and  the  slit  of  the  mouth 
is  distinct.  There  has  been  a  great  growth  of  the  regions  of  the  fore-brain  and 
mid-brain,  and  it  is  this  growth  chiefly  which  has  caused  the  relative  expansion 
of  the  head  as  compared  with  the  rest  of  the  body.  The  auditory  groove  now 
appears  distinctly  as  the  anlage  of  the  external  meatus  of  the  ear,  behind  which 
a  protuberance  can  be  seen  which  is  the  anlage  of  the  external  concha  of  the  ear. 
The  cervical  sinus  has  wholly  disappeared.  Along  the  line  of  the  back  the  primi- 
tive segments  are  scarcely  recognizable  in  the  cervical  region,  but  near  the  upper 
limb  they  still  show  distinctly  and  from  there  are  indicated  with  increasing  clear- 
ness as  we  pass  toward  the  lower  limb.  The  marks  of  the  segmental  divisions 
do  not  extend  as  far  on  the  dorsal  side  as  in  the  earlier  embryos,  but  are  re- 
stricted to  what  may  be  called  the  segmental  ridge.  Along  the  milk-line  a  series 
of  small,  white,  circular  spots  can  be  seen.  In  the  specimen  figured  there  were 
six  of  these;  their  number  is  variable.  They  are  the  anlages  of  the  mammary 
glands,  and  are  at  this  stage  merely  the  local  thickenings  of  the  ectoderm  or 
epidermis.  There  has  been  a  considerable  growth  of  the  dorsal  region  of  the 
body,  and  this  is  perhaps  most  clearly  indicated  by  the  position  of  the  milk-line, 
which  is  much  further  away  from  the  median  dorsal  line  than  in  the  10  mm.  pig- 
The  limbs  are  both  paddle-shaped,  and,  though  still  very  short,  have  a  broad 
terminal  expansion,  which  is  the  anlage  of  the  foot.  The  front  foot  has  some- 
what the  outline  of  a  truncated  pyramid,  while  the  hind  foot  is  more  rounded. 
In  the  anterior  limb  the  differentiation  into  upper  and  lower  divisions  is  sug- 
gested. 

Pig  Embryo  of  20  mm. — As  compared  with  figure  no  this  stage  (Fig.  in) 
shows  a  general  progress,  but  no  such  striking  changes  of  external  form  as 
distinguished  the  embryo  of  15  mm.  from  that  of  10  mm.  Embryos  of  this 
length  vary  considerably  in  their  proportions,  but  the  one  figured  is  charac- 
teristic of  this  stage.  The  enormous  transverse  diameter  of  the  body  as 
compared  with  its  length  is  very  striking,  and  the  very  large  size  of  the  head 
as  compared  with  the  body  is  almost  equally  remarkable.  In  the  head  the 
growth  of  the  regions  of  the 'fore-brain  and  mid-brain  has  continued,  and  the 


172 


STUD  Y  OF  PIG  EMBR  YOS. 


FIG.  in. — PIG  EMBRYO  OF  20  MM.     X  8  diams. 


EMBRYO  OF  12  MM.  STUDIED  IN  SECTIONS.  173 

divisions  between  the  mid-brain  and  hind-brain  are  marked  by  concavities  in  the 
outline  of  the  head.  The  eye  is  both  absolutely  and  relatively  larger.  Above  it 
can  be  distinguished  readily  the  anlages  of  the  great  bristles  which  develop 
over  the  eye,  corresponding  to  the  human  eyebrow.  These  anlages  appear 
as  whitish  spots,  for  they  are  thickenings  of  the  ectoderm.  The  snout  has 
increased  in  length ;  the  external  ear  has  grown  longer  and  has  begun  to  as- 
sume its  permanent  pointed  form.  The  limbs  have  increased  considerably  in 
length,  but  not  yetenough  to  pass  beyond  the  line  of  the  abdomen.  In  both 
feet  the  differentiation  of  five  toes  is  clearly  indicated.  The  milk-line,  as  a  line, 
has  almost  completely  vanished,  but  the  row  of  toes,  and  the  anlages  of  the 
mammary  glands,  which  develop  along  the  milk-line,  persist  and  will  undergo 
further  development  in  later  stages.  The  number  in  the  specimen  figured  is 
five.  A  row  of  these  anlages  marked  the  position  of  the  milk-line,  which 
demonstrates  that  there  has  been  a  more  marked  growth  of  the  dorsal  region  of 
the  body,  and  comparison  of  embryos  of  15  and  20  mm.,  during  the  period  com- 
prised between  those  two  stages,  indicates  that  the  growth  of  the  dorsal  part 
of  the  embryo  is  far  greater  than  of  the  ventral  part.  Comparison  of  figures 
1 10  and  1 1 1  shows  at  once  that  the  area  occupied  in  both  figures  by  the  region 
on  the  ventral  side  of  the  milk-line  is  about  the  same.  In  the  pig  of  20  mm.  there 
is  no  indication  of  the  segmental  structures  recognizable  in  the  surface  modeling. 

Pig  Embryo  of  12  mm.  Studied  in  Sections. 

A  pig  embryo  of  1 2  mm.  has  been  selected  as  the  center  of  study  in  this  boov 
because  its  anatomical  relations  are  such  that  they  may  be  readily  grasped  by 
the  student  who  has  already  studied  the  anatomy  of  an  adult  mammal,  human 
or  other.  At  the  same  time  the  development  of  the  organs  is  so  advanced  that 
their  fundamental  relations  may  be  observed.  From  an  embryo  of  this  size  the 
transition  to  the  study  of  younger  embryos  is,  even  for  the  beginner,  compara- 
tively easy.  It  is  not  necessary  that  the  embryo  should  be  of  this  exact  size; 
indeed,  it  may  be  somewhat  advantageous  for  the  student  to  have  an  embryo 
a  millimeter  larger,  or  one,  two,  or  even  three  millimeters  smaller,  since  the  fig- 
ures and  explanations  referring  to  the  12  mm.  stage  will  enable  him  to  identify 
all  the  structures  to  be  found  and  yet  call  upon  him  for  the  exercise  of  care  and 
judgment  in  identifying,  from  the  data  given  in  the  following  pages,  the  various 
parts  in  the  somewhat  different  stage  he  may  be  studying.  The  1 2  mm.  size  was 
chosen  partly  because  the  author  had  at  his  disposal  three  good  series  belonging 
to  the  Harvard  Embryological  Collection. 

The  transverse  series  is  the  most  important,  and  should  form  the  basis  of 
the  study,  and  accordingly  most  of  the  sections  figured  are  from  such  a  series. 


174 


STUDY  OF  PIG  EMBRYOS. 


Next  in  importance  comes  the  sagittal  series,  but  it  is  desirable  that  every  stu- 
dent should  have  a  series  in  the  three  standard  planes  at  his  disposal  for  study. 
In  the  practical  laboratory  study  each  student  should  be  required  to  make  a  series 
of  accurate  camera  lucida  drawings  of  carefully  selected  typical  sections ;  then  to 


28*       34-0  380 


i      .  112. — RECONSTRUCTION  OF  A  PIG  EMBRYO  OF  12.0  MM.  WITH  INDICATIONS  OF  THE  PLANES  OF  SEC- 
TIONS FIGURED. 
an,  Cloacal  membrane.     Ao,  Aorta.    Au,  Auricle.     A. urn,  Umbilical  artery.     a.z>,  Vertebral  artery,    has,  Basilar 

artery,     c,  Anlage  of  caecum,     car,  Internal  carotid,     cau,  Caudal  artery.      C.  Ex,  External  carotid  artery. 

f.b,   Fore-brain.     G,    Spinal   ganglion,     g,   Gall-bladder.     h.l>,   Hind-brain.      In,   Intestine.     Li,   Liver. 

in.b,  Mid-brain,     op,  Optic  vesicle.      Of,  Otocyst.     pan,  Pancreas.     Sp,  Spinal  cord.     St,  Stomach.      Um, 

Umbilical  opening.      Ven,  Ventricle  of  heart.     ///,  IV,  V,  Aortic  arches. 


185  Transverse  section,  Fig.  113. 


198 
249 
292 
321 
47° 
513 
633 


Fig.  114. 
Fig.  1 1 6. 
Fig.  117. 
Fig.  1 1 8. 
Fig.  119. 
Fig.  1 20. 
Fig.  121. 


284  Frontal  section,  Fig.  126. 
340        "  "        Fig.  127. 

380        "  "        Fig.  128. 

572        "  "        Fig.  129. 


TRANSVERSE  SECTIONS  OF  EMBRYO  OF  12  MM.  175 

name  correctly  all  the  parts  shown  in  each  section  and  to  identify  the  distribution 
of  the  three  germ-layers  in  every  case.*  A  sufficient  number  of  high-power 
drawings  ought  to  be  added  to  illustrate  the  character  of  the  various  tissues. 

The  accompanying  figure  112  represents  the  outline  of  the  pig  embryo  which 
was  cut  into  the  series  of  transverse  sections  from  which  figures  113  to  121  have 
been  made.  The  student  can  easily  identify  the  parts  in  the  figure  by  comparison 
with  that  of  the  pig  of  10  mm.  (Fig.  99),  aided  by  the  accompanying  description 
of  the  same.  The  sections  of  this  embryo  are  ten  mikrons-  in  thickness,  and  are 
966  in  number,  not  1 200,  as  the  student  might  expect.  The  discrepancy  is  due 
to  the  shrinkage  of  the  embryo  when  imbedded  in  paraffin.  The  shrinkage  is 
always  very  great,  and  in  the  case  of  embryos  causes  a  loss  of  almost  20  per  cent, 
in  the  length;  but  as  it  seems  to  take  pjace  uniformly  throughout  the  embryo, 
it  causes  no  distortion,  so  that  the  embryo  in  paraffin  is  an  exact  though  greatly 
reduced  copy — so  to  speak — of  the  living  embryo.  It  should  be  remembered 
that  no  correct  measurements  of  the  size  of  organs  or  cells  can  be  obtained  from 
sections  made  by  the  paraffin  method.  This  limitation  upon  the  use  of  sections 
is  too  often  forgotten.  The  horizontal  lines  indicate  approximately  the  levels 
at  which  the  sections  here  figured  belong.  For  convenience  the  direction  and 
position  of  the  frontal  sections  represented  in  figures  1 22  to  1 25  are  also  indicated 
approximately  on  this  same  outline,  although,  of  course,  the  frontal  series  was 
from  another  embryo.  , 

The  Study  of  Transverse  Sections. 

The  figures  and  descriptions  here  presented  of  eight  sections  have  i 
selected  as  illustrating  the  most  important  structures,  with  the  exception  of  the 
disposition  of  the  umbilical  opening  and  of  the  kidney,  which  can  be  better  repre- 
sented in  sections  from  older  or  younger  stages.         * 

Section  through  the  Upper  Part  of  the  Ol(\cyst. — As  shown  in  figure  1 12,  by  the 
line  185,  this  section  is  taken  from  a  level  about  half-way  between  the  eye  and 
the  highest  point  in  figure  112.  It  passes,  therefore,  through  the  fore-brain, 
F.  b,  and  the  fourth  ventricle,  Ven.  IV,  or  cavity  of  the  hind-brain.  The  section 
is  bounded  by  a  thin  layer  of  epidermis,  between  which  and  the  brain-wall  there 
is  a  large  amount  of  mesenchymal  tissue.  Alongside  the  hind-brain  lies  a 
of  important  structures  imbedded  in  tiie  in.  ma,  which  are  identical 

the  two  sides  Although  they  differ  somewhat  in  the  section,  as  the  plane  o 
ting  was  not  symmetrically  transverse  for  the  head.     These  structures  a 


*  For  making  camera  lucid,.  J-inch  objective  will  be  found  convenient.      An  Abbe 

is  recommended.     Compare  the  directions  fo.  Drawing  in  Chapter  VIII. 


176 


STUDY  OF  PIG  EMBRYOS. 


following  order:  N.  5,  the  trigeminal  ganglion;  N.  7,  8,  the  acustico-facial  gan- 
glion complex;  Ot,  the  otocyst,  an  oval  vesicle  with  very  distinct  epithelial  walls. 


com.  II. 


Ven.  IV. 


FIG.  113. — PIG,  12.0  MM.     TRANSVERSE  SERIES  5,  SECTION  185. 

com,  Ganglionic  commissure  between  hypoglossai  and  vagus  nerves.  com.  I  I,  Ganglionic  commissure  with 
fibers  of  the  root  of  the  spinal  accessory  nerve.  D.E,  Ductus  endolymphaticus.  /•'.£,  Fore-brftin,  Md, 
Medulla  oblongata.  jV.j,  Trigeminal  ganglion.  N,j,8,  Acustico-facial  ganglion.  X.io,  Jugular 
ganglion  of  the  vagus.  Ot,  Otocyst.  Ven. IV,  Fourth  ventricle.  X  22  diams. 


Next  the  ninth  or  glosso-pharyngeal  nerve  (scarr  e'jy  appearing  in  the  section  on 
the  right  side  of  the  embryo)  is  shown  by  the  upper  portion  of  its  ganglion  on  the 


TRANSVERSE  SECTIONS  OF  EMBRYO  OF  12  MM.  177 

left  side  (the  right  in  the  figure) ;  the  ganglion  is  a  small,  dark  mass  of  triangu- 
lar outline  close  to  the  medullary  wall  of  the  hind-brain,  and  it  lies  almost  even 
with  the  posterior  edge  of  the  otocyst.    A".  10  is  the  vagus  nerve  (the  section  passes  / 
through  the  upper  portion  of  the  jugular  ganglion  of  that  nerve) ;  it  shows  better 
on  the  right  side  of  the  embryo  in  this  section  than  on  the  left.  Com.  10,  and  com, 
refer  to  the  ganglionic  commissure  which  extends  above  the  origins  of  the  hypo- 
glossal  nerve.     On  the  left  the  continuity  of  this  commissure  is  better  shown 
than  on  the  right,  where  it  offers  two  parts,  one,  com,  entirely  ganglionic,  and 
another,  com.  10,  which  comprises  both  the  ganglionic  portion  and  fibers  which 
share  in  the  formation  of  the  root  of  the  tenth  nerve.     The  trigeminal  ganglion 
is  very  large,  somewhat  triangular  in  shape,  the  apex  of  the  triangle  joining  the 
angle  of  the  hind-brain.     This  situation  is  very  characteristic,  for  the  trigeminal 
is  always  at  this  angle,  and,  from  its  great  size  and  position,  is  one  of  the  most 
important  landmarks  in  the  study  of  the  topography  of  the  embryonic  head. 
The  nerve-cells  of  the  ganglion  are  grouped,  for  the  most  part,  on  the  side  toward 
the  ectoderm,  where  they  are  closely  crowded  together,  making  a  deeply  staining 
mass.     Nearer  the  brain- wall  the  tissue  of  the  ganglion  is  much  less  condensed, 
is  somewhat  penetrated  by  small  blood-vessels,  and  contains  a  considerable 
number  of  nerve-fibers,  which  are  gathered  into  small  bundles.     Toward  the 
brain-wall  the  bundles  become  distinct,  and  on  the  right  side  of  the  embryo  the 
passage  of  the  nerve-fibers  into  the  brain  can  be  readily  seen.     The  nerve-libers 
at  this  stage  are  merely  neuraxons,  that  is  to  say,  merely  thread-like  prolonga- 
tions of  the  bodies  of  the  nerve-cells  (neurones).     The  fibers  are  entirely  witi 
sheaths.     They   stain   very  lightly,   and   hence,   in  the  preparation,  mB 
detected  by  their  light  appearance.     The  nerve-fibers  may  be  convciueiith 
dered  conspicuous  by  counterstaining  the  sectioi  s  with  I/yens  blue.     The  JH^I 
fibers  of  the  trigeminus,  which  enter  the  wall  of  1Ji£  Innd-brain,  form  in 
bundle  of  fibers,  wh''  h  extends  along  past  the  acustico-facial  ganglia  v 
medullary  wall.     These  fibers  represent  the  commencement  of  the  ascending 
trigeminu*  tract  of  anatomists.     The  other  ganglia  associated  with  the  hind-brain 
are  not  well  shown  in  this  section.     The  otocyst  has  a  very  sharply  defined  epi- 
thelial wall  and  is  imbedded  in  loose  mesenchymal  tissue.     On  the  right  side  of 
the  embryo  we  have  the  ductus  endolymphaticus,  D.E,  the  opening  of  whicii  into 
the  main  cavity  of  the  otocyst  is  shown  on  the  left  side.     The  epithelial  wall  < 
the  ductus  is  thicker  than  that  of  the  greater  part  of  the  otocyst  proper^ 
wall,  Md,  of  the  hind-brain  exhibits  already  characteristic  differentiations,  for 
it  shows  clearly  the  three  primitive  layers;  the  outer  o1-  external  rieuroglia  layer  ^ 
is  thin,  and  appears  light  in  the  section  because  it  takes  the  stain  slightly! 
in  this  outer  neuroglia  layer  (ectoglia)  that  the  entire  sensory  nerve-fibers  are 


178  STUDY  OF  PIG  EMBRYOS. 

primarily  distributed,  and,  therefore,  it  is  in  a  portion  of  this  layer  that  we  find 
the  ascending  trigeminal  tract  situated.     Next  to  the  ectoglia  comes  the  middle 
layer,  in  which  the  neurones  of  the  medullary  wall  are  situated,  and  which  is, 
therefore,  termed  the  neurone  or  gray  layer  (cinerea),  easily  recognizable  under 
the  microscope  by  its  brighter  color,  which  is  due  principally  to  the  fact  that  the 
nuclei  in  this  layer,  though  numerous,  are  much  less  crowded  than  in  the  inner- 
most of  the  three  layers,  or  primitive  ependymal  layer,  which  at  this  stage  is 
quite  thick.     Owing  to  the  presence  of  nuclei,  the  gray  layer  is,  of  course,  stained 
much  more  than  the  ectoglia.     The  nuclei  of  the  brain-wTall  show  as  yet  very 
little  differentiation.     There  "are  numerous  mitotic  figures  which  are  situated 
exclusively  close  to  the  inner  surface  of  the  brain- wall  in  the  fore-brain.     The 
structure  of  the  fore-brain  is  similar,  but  the  development  is  less  advanced ;  the 
differentiation  of  the  neurone  layer  is  only  just  beginning,  and  it  has  acquired 
little  thickness.     In  the  hind-brain  we  see  in  the  interior,  along  the  region  be- 
tween the  otocysts,  a  series  of  curved  notches  which  impart  a  scalloped  outline  to 
the  wall.     A  distinct  point  separates  one  concavity  from  the  next.     Each  one 
of  the  spaces  between  two  of  the  projecting  points  is  designated  as  a  neuromere. 
The  neuromeres  correspond  in  number  and  position  to  the  neighboring  primitive 
segments,  and  are,  therefore,  to  be  designated  as  segmental  structures.     They 
also  bear  an  evident  relation  to  the  development  of  the  nerves,  and  the  accepted 
hypothesis  is  that  from  each  neuromere  springs  a  single  nerve.     The  attempts 
which  have  been  made  to  verify  this  hypothesis  have  met  with  very  serious  diffi- 
culties, for  the  relations  are  extremely  complicated,  and  until  the  matter  shall 
been  much  more  thoroughly  investigated  than  at  present,  we  must  remain 
in  the  dark  as  to  the  precise  morphological  value  of  the  neuromeres.     But,  inas- 
much as  they  appeal  with  the  greatest  constancy  in  the  embryos  of  all  verte- 
brates, we  cannot  help  accepting  fl.e  view  that  they  are  really  structures  of  fun- 
damental importance.   At  the  stage  we  are  st  !!'1  ving  the  neuromeres  have  already 
begun  to  lose  their  distinctness,  and  in  slightly  older  pigs  can  be  traced  only  with 
difficulty.     In  younger  stages  their  primitive  characteristics  are  better  shown 
(compare  page  229).     As  regards  the  blood-vessels  in  the  present  section :  there 
are  small  branches  of  the  veins,  which  show  outside  of  the  ganglionic  commis- 
sure, com;  parts  of  the  jugular  vein  appear  in  close  proximity  to  the  trigeminal 
ganglion,  and  again  at  the  side  of  the  head.     In  the  median  line  between  the  fore- 
brain  and  hind-brain,  or  nearer  to  the  layer,  appears  a  section  of  the  basilar 
artery.     Near  the  fore-brain  on  either  side  is  the  loop  of  thef  carotid  artery.     There 
are  several  important  points  to  be  observed  in  the  region  between  the  trigeminal 
ganglia  and  the  fore-brain.     In  order  to  show  these  more  clearly,  a  separate  illus- 
tration (Fig.  1 14),  on  a  larger  scale,  of  this  portion  of  the  section  is  given.     The 


TRANSVERSE  SECTIONS  OF  EMBRYO   OF  12  MM. 


179 


trigeminal  ganglion,  the  wall  of  the  fore-brain,  and  the  wall  of  the  hind-brain  will 
be  at  once  identified,  so  that  the  correspondence  with  the  general  figure  is  easily 
followed.  Between  the  trigeminal  ganglion  and  the  fore-brain  are  four  veins, 


FIG.  114. — PORTION  OF  FIG.  113  MORE  HIGHLY  MAGNIFIED. 

A. has,  Basilar  artery.  A. car,  Internal  carotid  artery.  Arach,  Arachnoid  zone.  Cm,  Neurone  layer  of  medulla. 
Cut,  Cutis  layer.  Ec.gl,  Ectoglia.  Epen,  Ependymal  layer.  G.tri,  Trigeminal  ganglion.  Jug',  J'ig''  •> 
Jugf",  Jugular  vein.  Jug.  I.,  Lateral  branch  of  the  jugular.  N-3,  Oculomotor  nerve.  N-4,  Trochlear 
nerve.  -A7-?,  Sensory  root  of  trigeminus.  JV.?,8,  Acustico-facial  ganglion.  Pia,  Pia  mater.  Raph, 
Raphe  of  the  medulla  oblongata.  Veii.III,  Third  ventricle  of  the  brain.  Ven.IV,  Fourth  ventricle  of  the 
brain.  X  5°  diams. 

two  of  which,  Jug'  and  Jug'" ,  are  larger  and  are  parts  of  the  main  jugular  stem 
passing  from  the  region  of  the  hind-brain  to  that  of  the  fore-brain,  while  the  two 
smaller  ones,  Jug" ,  are  merely  branches  of  the  jugular.  Close  to  the  section, 


• 

180  STUDY  OF  PIG  EMBRYOS. 

Jug'" ,  of  the  jugular  nearest  the  fore-brain  lie  the  very  small  sections  of  the 
fourth,  N.4,  and  third,  N.j,  cerebral  nerves.  The  fourth  nerve  is  minute  in  size 
and  lies  just  behind  the  jugular.  The  third  nerve,  though  somewhat  larger,  is 
also  very  small  and  lies  anterior  to  the  jugular  somewhat  on  its  medial  side.  Both 
of  these  nerves,  owing  to  their  small  dimensions,  are  somewhat  difficult  to  ob- 
serve with  the  low  power.  The  detailed  figure  brings  out  more  clearly  other 
points.  It  shows  very  clearly  the  junction  of  the  trigeminal,  N.  5,  and  acustico- 
facial,  N.  7,  8,  ganglia  with  the  wall  of  the  hind-brain,  and  also  the  division  of 
that  wall  into  its  three  primary  layers,  the  ectoglia,  EC.  gl,  the  gray  layer,  cin,  and 
the  inner  or  ependymal  layer,  Epen,  and  also  the  median  floor-plate  or  raphe, 
Raph.  Immediately  below  it  is  the  basilar  artery.  On  either  side  of  the  fore- 
brain  is  the  section  of  the  loop  of  the  carotid,  A.  car,  which  is  passing  up  to  join 
the  anterior  end  of  the  basilar  artery,  which  last  has  been  produced  by  the  fusion 
of  the  two  originally  symmetrical  vertebral  arteries.  This  portion  of  the  carotid 
loop  probably  corresponds  to  the  vessel  designated  in  the  adult  as  the  posterior 
communicating  branch,  by  which  the  end  of  the  carotid  proper  anastomoses  with 
the  basilar  artery.  At  the  side  of  the  fore-brain  appears  a  blood-vessel,  Jug.  L, 
which  might  be  called  the  lateral  jugular.  It  is  a  branch  of  the  main  jugular 
stem  and  passes  over  the  side  of  the  fore-brain  toward  the  median  dorsal  surface 
thereof,  where  it  meets  the  corresponding  vein  of  the  opposite  side,  with  which  it 
then  unites  to  form  a  single  median  vessel.  This  vessel  ultimately  acquires 
great  size,  and  is  known  as  the  superior  longitudinal  sinus.  It  is  shown  in  figure 
105.  So  much  of  the  vessels  as  do  not  unite  in  the  median  line  persist,  to  form  the 
lateral  sinus  of  the  adult.  These  sinuses  in  the  embryo  are  all  small  branches  of 
the  veins  when  they  first  appear.  Their  great  enlargement  does  not  occur  until 
comparatively  advanced  stages.  Finally,  attention  should  be  paid  to  the  follow- 
ing imp/ortant  modifications  in  the  mesenchyma.  Already  there  has  been  a  rich 
development  of  a  plexus  of  fine  blood-vessels  over  the  surface  of  both  the  fore- 
nd  hind-brain  which  has  been  accompanied  by  a  slight  condensation  of  the  mes- 
enchyma between  the  blood-vessels,  thus  marking  a  distinct  membrane,  in  which 
we  can  easily  recognize  the  pia  mater,  Pia.  Outside  of  the  pia  mater  conies  a 
relatively  broad  zone,  Arach,  in  which  the  cells  are  widely  separated  from  one 
another  and  are  connected  by  slender  and  long  processes  so  that  the  intercellular 
spaces  are  very  extensive.  This  broad  zone  is  the  anlage  of  the  arachnoid  mem- 
brane. It  is  much  more  differentiated  around  the  ventral  portion  of  the  brain 
than  around  the  dorsal  side.  Between  the  arachnoid  zone  and  the  external 
epidermis  the  mesenchyma  is  somewhat  more  condensed  and  the  cells  are  elon- 
gated in  form,  in  part  almost  spindle-shaped,  forming  a  layer,  Cut,  which  we  may 
consider  the  anlage  of  the  cutis,  and  perhaps,  also,  of  the  subcutaneous  tissue. 


TRANSVERSE  SECTIONS  OF  EMBRYO   OF  12  MM.  181 

but  this  is  doubtful.  Between  the  arachnoid  zone  and  the  cutis  zone,  so  placed 
that  they  cannot  be  quite  said  to  belong  to  either  one  or  the  other,  appear  numer- 
ous blood-vessels.  These  form  a  more  or  less  distinct  vascular  layer,  which 
appears  with  remarkable  constancy  in  all  classes  of  vertebrates,  and  over  a  large 
part  of  the  body.  It  may,  therefore,  be  called  the  panchoroid.  It  is  unques- 
tionably of  very  great  morphological  importance,  but  its  history  is  almost  un- 
known. 

As  regards  the  histological  condition  of  the  tissues  the  student  should  make 
careful  observations.  Attention  may  be  directed  especially  to  the  following 
points:  The  epidermis  at  the  sides  of  the  section  is  two-layered  and  consists  of- 
an  inner  layer  of  cuboidal  cells,  the  anlage  of  the  Malpighian  layer  of  the  adult, 
and  of  an  outer  layer  of  very  thin  cells,  the  epitrichium,  the  nuclei  of  which  are 
flattened  and  appear  darkly  stained.  Toward  the  median  line,  above  the  hind- 
brain  and  below  the  fore-brain,  the  epidermis  becomes  gradually  one-layered 
and  much  thinner.  The  mesenchyma  exhibits  three  principal  forms  of  cells: 
First,  those  which  are  equally  branched  in  all  directions  and  represent  a  primitive 
form  of  the  tissue.  Such  may  be  found  in  the  neighborhood  of  the  basilar  artery. 
Second,  the  elongated  cells  of  the  cutis  zone;  and,  third,  the  cells  of  the  arach- 
noid zone  above  described.  The  blood-vessels  have  very  distinct  endothelial 
walls  which  are  very  thin,  being  thickened  only  to  furnish  space  for  the  nuclei, 
which,  unlike  those  of  the  adult,  project  not  only  into  the  lumen  of  the  vessel, 
but  also  against  the  surrounding  mesenchyma.  The  blood-corpuscles  are 
rounded  cells,  sometimes  oval,  not  infrequently  somewhat  distorted.  Their 
nuclei  are'  nearly  spherical  and  contain  a  number  of  tine  granules.  Mitotic 
figures  are  quite  frequent.  A  few  of  the  nuclei  are  beginning  to  change  by  be- 
coming smaller  and  taking  the  stain  more  deeply  (compare  page  94).  In  the 
nervous  system  the  differentiation  of  the  cells  in  the  hind-brain  is  more  advanced 
than  in  the  fore-brain,  but  even  in  the  hind-brain  the  distinction  between  the 
young  nerve-cells  and  the  young  neuroglia  cells  (neuroblasts  and  spongioblasts) 
is  not  very  clear.  The  nuclei  are  only  just  beginning  to  acquire  distinct  nucleoli, 
such  as  would  be  characteristic  of  later  stages.  The  nuclei  of  the  tissues  differ 
markedly  from  those  of  the  earliest  embryonic  stages,  but  can  scarcely  be  said  to 
have  assumed  in  any  of  the  tissues  adult  characteristics. 

Section  through  the  Lower  Part  of  the  Otocyst. — Figure  1 1 5  is  from  section  198, 
and,  therefore,  ten  sections  below  figure  113.  It  is  inserted  chiefly  to  bring  out 
three  points  not  shown  in  the  preceding  illustration :  First,  the  root  of  the  spinal 
accessory  nerve,  N.  //,  which  arises  from  the  cervical  (in  the  figure  upper)  end  of 
the  hind-brain  and  runs  forward  to  join  the  vagus  ganglion,  N.  10  jug,  the  jugu- 
lar ganglion  cf  the  adult.  Second,  the  characteristic  relations  of  the  jugular  vein 


182 


STUDY  OF  PIG  EMBRYOS. 


to  the  trigeminal  ganglion,  N.  5.     The  vein  is  cut  twice,  Jug'  and  Jug",  for  it 
curves  around  the  ganglion,  passing  on  the  inside  of  the  ganglion  between  it  and 


N.I  i. 


Sir. 


Ven.iv. 


Me/. 


EC. 


FIG.  115. — PIG,  12.0  MM.     TRANSVERSE  SERIES  5,  SECTION  198.  % 

EC,  Ectoderm.  F.b,  Fore-brain.  In/,  Infundibular  gland.  Jug' ,  Jug" ,  Jugular  vein.  N.$,  Trigeminal  gan- 
glion. N.7,8,  Acustico-facial  ganglion.  N. g,  Ganglion  nodosum  of  the  glossopharyngeal  nerve.  N.io. 
jug,  Jugular  ganglion  of  the  vagus  nerve.  N.n,  Root  of  the  spinal  accessory  nerve.  Md,  Medulla  oblon- 
gata.  Ot,  Otocyst.  Str,  Striae  acusticte.  Ven.iii,  Third  ventricle.  Ven.iv,  Fourth  ventricle.  X  22  diams. 

the  wall  of  the  brain.     At  this  stage,  however,  it  has  begun  to  expand  outward 
behind,  in  front  of,  and  below  the  ganglion.     The  expansion  of  the  vein  outward 


TRANSVERSE  SECTIONS  OE  EMBRYO   OF  12  MM.  183 

will  continue  through  later  stages  until  a  new  venous  path  is  established  on  the 
outside  of  the  trigeminal  ganglion,  which  will  then  appear,  so  to  speak,  as  an 
island  in  the  path  of  the  jugular.  Still  later  the  vein  on  the  inner  side  of  the  tri- 
geminal ganglion  will  abort,  and  only  the  external  pathway  will  be  retained.  By 
these  changes  the  vein  migrates  from  its  original  internal  position  to  its  new  per- 
manent external  position.  In  the  1 2  mm.  pig  the  jugular  vein  pursues  a  very  sin- 
uous course  along  the  sides  of  the  hind-brain,  for  it  passes  inside  of  the  twelfth, 
eleventh,  tenth,  and  ninth  nerves,  then  outside  of  the  otocyst,  the  seventh  and 
eighth  nerves,  and  inside  of  the  fifth.  In  an  earlier  stage  it  lay  inside  of  the  oto- 
cyst, but  is  now  found  to  have  migrated  to  the  outside  thereof,  by  the  process  of 
island  formation  just  described  for  the  trigeminus.  The  blood-vessel  in  its  origi- 
nal condition  is  properly  termed  the  anterior  cardinal.  When  it  has  completely 
migrated  outside,  not  only  of  the  otocyst  and  of  the  trigeminal  ganglion,  but  also 
outside  of  the  glosso-pharyngeal  and  vagus  nerves,  it  may  properly  be  termed 
the  jugular.  The  jugular,  therefore,  is  to  be  defined  as  the  anterior  cardinal  vein 
which,  by  successive  island  formations,  has  migrated  to  a  new  position  outside  of 
the  otocyst  and  cephalic  ganglia.  Third,  to  show  the  infundibular  gland,  Inf,  a 
small  evagination  from  the  ventral  floor  of  the  fore-brain,  F.b.  The  evagination 
is  really  hollow,  but  the  cavity  does  not  appear  in  the  section  figured.  It  enters 
into  very  close  relations  with  another  hollow  evagination,  which  springs  from  the 
dorsal  roof  of  the  oral  cavity  and  is  known  as  the  hypophysis.  The  infundibu- 
lar gland  and  the  hypophysis  become  intimately  associated  with  one  another  in 
their  further  development  and  give  rise  to  the  pituitary  body  of  the  adult,  the 
gland  becoming  the  posterior  lobe,  the  hypophysis  the  anterior  lobe  of  that  organ. 
The  hypophysis  may  be  best  studied  in  sagittal  sections  (see  page  205).  The 
present  section,  figure  115,  being  at  a  lower  level  than  figure  113,  passes  through 
the  ventral  portion  of  the  hind-brain  and  shows  only  a  narrow  part  of  the  cavity 
of  the  fourth  ventricle,  Ven.ii).  The  three  layers  in  the  wall,  Md,  of  the  hind-brain 
are  very  distinct.  At  the  anterior  end  of  the  hind-brain  appears  a  series  of  light 
lines,  Sir,  which  are  caused  by  nerve-fibers.  These  lines  have  been  identified  as 
the  stria  acusticce.  They  need  to  be  more  accurately  studied,  however,  for  they 
seem  rather  to  be  fibers  of  the  lateral  root  of  the  facial  nerve.  Close  to  the  ante- 
rior section  of  the  jugular  vein,  Jug',  appear  the  minute  fourth  and  third  nerves, 
which,  however,  are  not  indicated  in  the  figure.  Both  lie  close  to  the  wall  of  the 
vein  on  the  side  away  from  the  trigeminal  ganglion.  The  fourth  nerve  lies 
nearer  the  outside  of  the  embryo,  the  third  nerve  nearer  the  median  plane. 
At  about  the  same  level  as  this  part  of  the  jugular  vein,  and  very  close  to  the 
wall  of  the  fore-brain,  is  situated  the  loop  of  the  internal  carotid.  Lower  down, 
but  not  close  to  the  wall  of  the  fore-brain,  is  the  section  of  the  lateral  jugular. 


184  STUDY  OF  PIG  EMBRYOS. 

Section  through  the  First  Gill  Cleft  and  Optic  Evaginations . — The  section 
shows  on  the  dorsal  side  the  upper  cervical  region  of  the  spinal  cord,  on  the  ven- 
tral side  the  fore-brain  giving  off  the  optic  nerves.  In  this  and  the  three  sections 
next  following  the  complicated  pharynx  appears  in  various  forms.  The  general 
shape  of  the  pharynx  has  been  described  with  the  aid  of  a  figure  of  a  wax  model 
of  the  pharynx  made  from  the  same  series  of  sections  from  which  these  figures  are 
taken.  The  shape  of  the  pharynx  and  of  its  four  pairs  of  lateral  pouches  at  this 
stage  is  remarkably  constant,  so  the  student  is  not  likely  to  encounter  any  serious 
difficulty  in  identifying  the  parts.  The  spinal  cord  is  oval  in  the  section.  Its 
cavity  has  expanded  in  the  middle.  The  lateral  walls  are  quite  thick,  the  median 
ventral  wall  is  thinner,  and  the  median  dorsal  wall  (deck-plate)  is  very  thin. 
The  three  primitive  layers  of  the  medullary  tube  are  very  clearly  marked  out,  tfie 
ectoglia  appearing  light,  the  ependymal  layer  appearing  dark.  The  differentia- 
tion is  much  more  advanced  on  the  ventral  side  of  the  spinal  cord  than  on  the 
dorsal  side,  and,  indeed,  it  is  only  in  the  ventral  part  that  the  three  layers  are 
perfectly  differentiated.  In  the  median  ventral  line  we  have  the  floor-plate,  in 
which  we  can  distinguish  only  two  zones,  while  in  the  deck-plate  there  is  no 
differentiation  of  layers  whatever.  The  spinal  cord  is  clearly  divided  into  a 
dorsal  zone,  D.  Z,  and  a  ventral  zone,  V.  Z,  on  each  side.  The  two  dorsal  zones 
are  connected  across  the  median  line  by  the  thin  deck-plate,  and  the  ventral 
zones  similarly  by  the  thin  floor-plate.  The  lower  or  ventral  limit  of  the  dorsal 
zone  is  marked  by  the  entrance  of  the  dorsal  or  ganglionic  root  and  by  the  fibers, 
which  represent  the  outgoing  lateral  roots.  In  the  actual  section  figured,  the 
lateral  roots,  L.  R.  //,  are  those  which  enter  into  the  formation  of  the  eleventh 
nerve.  The  true  dorsal  root  does  not  appear  in  the  figure  .  Internally  the  divi- 
sion between  the  two  zones  is  marked  by  the  lateral  angle  of  the  central  cavity 
shown  in  the  section.  In  the  dorsal  zone  the  differentiation  of  the  three  layers 
has  made  slight  progress.  In  the  ventral  zone,  however,  the  development  is  far 
more  advanced.  The  most  characteristic  feature  of  this  movement  is  the  growth 
of  the  cinerea  or  neurone  layer,  which  increases  in  a  twofold  manner:  first,  by 
encroaching  upon  the  inner  or  ependymal  layer;  and,  secondly,  by  the  growth 
of  its  constituent  elements.  Examination  with  a  high  power  shows  at  once  that 
the  cells  have  grown  very  much.  Their  nuclei  are  larger,  granular  in  appear- 
ance, rarely  with  any  indication  of  a  distinct  nucleolus.  Most  of  the  cells  are 
neuroblasts  and  have  well-marked  protoplasmic  bodies,  finely  granular  in  tex-' 
ture.  They  have  many  of  them  already  produced  long,  slender  outgrowths 
which  we  can  identify  as  the  neuraxons.  In  order  to  study  the  distribution  of 
the  neuraxons  and  the  form  of  the  neuroblasts,  it  is  necessary  to  apply  the  Golgi 
rapid  method,  by  which  it  can  be  demonstrated  that  a  portion  of  the  neuraxons 


TRANSVERSE  SECTIONS  OF  EMBRYO  OF  12  MM. 


185 


is  distributed  entirely  within  the  medullary  wall,  while  another  portion  passes 
out  to  form  ventral  roots,  one  of  which,  N.  12,  forming  part  of  the  hypoglossal 


D.z. 


v.z. 


'  L.R.i i. 


L.V. 


H. 


—  EC. 


FIG.   116. — PIG,  12.0  MM.     TRANSVERSE  SERIES  5,  SECTION  249. 

Car. in,  Internal  carotid  artery.  cl.I,  First  or  auditory  gill  cleft.  D.  Z,  Dorsal  zone  of  spinal  cord.  EC, 
Ectoderm.  //,  Anlage  of  cerebraj  hemisphere.  Jug,  Jugular  vein.  Jug.in,  Internal  jugular  vein. 
L,  Lens.  L.R.u,  Lateral  root  of  the  eleventh  nerve.  L.V,  Lateral  ventricle.  Mx.in,  Inferior  max- 
illary nerve.  N.j,  Facial  nerve.  N.q.petr,  Petrosal  ganglion  of  the  ninth  nerve.  N.io.n,  United 
vagus  and  spinal  accessory  nerve.  N.i2,  Hypoglossal  nerve.  Op,  Stalk  of  the  optic  evagination. 
Ph,  Pharynx.  Ret,  Retina.  V.Z,  Ventral  zone  of  spinal  cord.  X  22  diams. 

nerve,  is  shown  in  the  figure.     A  third  portion  of  the  neuraxons,  at  least  in  the 
upper  cervical  region,  as  also  in  the  medulla  oblongata,  passes  out  to  form  the 


186  STUDY  OF  PIG  EMBRYOS. 

lateral  roots.  The  positions  of  the  exits  of  these  two  bundles  of  nerve-fibers  are 
constant  and  characteristic.  The  ventral  root  always  passes  out  from  the  middle 
of  the  ventral  zone  about  half-way  between  the  median  floor-plate  and  the  dorsal 
limit  of  the  zone.  The  lateral  root  always  passes  out  at  the  upper  dorsal  limit  of 
the  ventral  zone  and  immediately  below  the  point  of  entrance  of  the  true  dorsal 
root.  Formerly  the  lateral  roots  were  not  distinguished  from  the  dorsal  roots. 
Following  downward  in  the  figure  we  come  to  the  section  of  the  jugular  vein,  Jug, 
just  inside  of  which  lies  the  common  trunk,  N.  10.  //,  of  the  united  tenth  and 
eleventh  or  vagus  and  accessorius  nerves,  and  also,  nearby,  the  lower  part  of  the 
petrosal  ganglion,  TV.  p.  petr,  of  the  glosso-pharyngeal  nerve.  Lower  down  and 
nearer  the  ectoderm  lies  the  facial  nerve,  N.  7,  situated  in  what  is  called  the  hyoid 
arch  or  mass  of  tissue  intervening  between  the  first  and  second  gill  cleft.  The 
hyoid  arch  is  further  marked  by  a  bulge  in  the  external  outline  of  the  section, 
which  leads  down  into  a  deep  groove  beyond  which  the  outline  of  the  section 
again  rises  and  arches  forward  to  the  eye.  This  groove  is  the  external  depres- 
sion of  the  first  gill  cleft  and  ultimately  is  transformed  into  the  external  auditory 
meatus.  The  position  of  this  groove  is  well  shown  in  figure  99,  Au,  on  page  160. 
Just  inside  the  auditory  groove  appears  the  outer  end  of  the  first  or  audi- 
tory internal  gill  pouch,  cl.  I.  It  is  a  long,  oblique  slit,  quite  narrow,  and  is 
lined  by  a  layer  of  entoderm.^'  If  we  follow  it  along  through  several  sections,  we 
shall  find  that  higher  up  its  outer  end  comes  in  contact  with  the  ectoderm  at 
the  bottom  of  the  auditory  groove,  and  there  the  two  germ-layers,  entoderm 
and  ectoderm,  unite  to  form  a  single  membrane,  the  closing  plate  of  the  gill 
pouch.  Following  through  the  section  downward  in  the  series,  we  can  trace 
the  cleft  to  its  connection  with  the  pharynx,  Ph.  On  the  posterior  side  of 
the  cleft  we  find  the  internal  carotid,  Car.  in.  Only  the  roof  of  the  pharynx, 
Ph,  is  cut,  so  that  it  occupies  but  a  small  area  in  the  section.  On  its  anterior 
side  it  shows  a  small  knob-like  projection  toward  the  floor  of  the  fore- 
brain.  This  is  a  part  of  the  stalk  of  the  hypophysis.  Below  the  hypophysis 
appears  the  very  large  and  conspicuous  inferior  maxillary  nerve,  MX.  in,  and 
beneath  that  the  section  of  the  small  internal  jugular  vein,  Jug.  in.  The  fore- 
brain  is  quite  complicated  in  shape,  having  two  lateral  expansions,  L.  V,  of  its 
cavity  which  are  destined  to  form  the  lateral  ventricles.  The  walls,  H,  of  the 
lateral  ventricles  are  the  anlages  of  the  cerebral  hemispheres.  From  the  ventral 
(in  the  figure  upper)  part  of  the  fore-brain  spring  on  either  side  the  optic  stalks, 
Op.  These  are  hollow  prolongations  of  the  brain,  which  expand  at  their  distal 
ends  to  form  the  retina  of  the  eye,  Ret,  and  the  pigment  layer.  The  expanded 
distal  ends  constitute  each  a  sort  of  cup,  of  which  the  optic  stalk  is  the  stem.  The 
cup  is  two-layered,  the  space  between  the  two  layers  being  a  prolongation  of  the 


TRANSVERSE  SECTIONS  OF  EMBRYO  OF  12  MM.  187 

central  cavity  of  the  brain.  The  inner  of  the  two  layers  forms  the  retina  proper 
and  is  considerably  thickened.  The  outer  layer  is  quite  thin  and  is  already 
quite  abundantly  laden  with  pigment  granules.  At  the  edge  of  the  cup  the  pig- 
ment layer  passes  over  uninterruptedly  into  the  thick  retina  layer.  In  the  cav- 
ity of  the  optic  cup  lies  the  vesicular  lens,  L,  which  arose  from  an  evagination  of 
the  overlying  ectoderm.  The  vesicle  is,  however,  now  completely  separated 
from  the  layer  which  produces  it.  It  has  at  this  stage  a  very  large  cavity,  and  in 
adjacent  sections  it  can  be  readily  seen  that  the  inner  side  or  that  toward  the 
brain  is  already  thickening  and  changing  its  character  so  as  to  form  the  main  part 
of  the  adult  lens.  The  thickening  depends  chiefly  upon  the  rapid  and  enor- 
mous elongation  of  the  epithelial  cells  of  this  part  of  the  vesicle,  so  that  they  are 
transformed  into  the  so-called  fibers  of  the  adult  lens,  each  adult  fiber  being  a 
single  epithelial  cell. 

Section  through  the  Second  Gill  Cleft  and  Oral  Fissure. — The  level  of  this  sec- 
tion is  such  that  the  head  is  cut  separately  and  appears  in  section  without  connec- 
tion with  the  body  of  the  embryo.  The  space  between  the  head  piece  and  body 
piece,  O.  F,  may  be  designated  as  the  oral  fissure,  since  it  is  into  this  space  that 
the  mouth  opens.  In  general  there  is  considerable  resemblance  between  this  and 
the  section  last  described,  but  in  the  present  section  the  eyes  have  disappeared 
and  we  get  the  first  indications  of  the  nasal  pits,  Olf.  That  on  the  left  side  of  the 
body  shows  a  trace  of  the  cavity  of  the  pit.  The  posterior  part  of  the  pharynx, 
Ph,  is  cut  in  the  section  instead  of  the  anterior  part,  as  in  figure  1 16.  The  first 
gill  cleft  does  not  show,  but  the  second  cleft,  cl.  II,  does.  It  lies  posterior  to  the 
first  cleft  and,  therefore,  appears  higher  up  in  the  figure.  The  spinal  cord,  Sp.  c, 
shows  the  same  general  structure  as  in  the  previous  section.  On  either  side  of  it 
may  be  seen  the  small  and  inconspicuous  root  of  the  eleventh  or  accessory  nerve. 
1 1  could  not  be  properly  represented  in  the.  Some  distance  below  the 

cord  lies  the  small  circular  section  of  the  notochord,  wnicL  differs  so  slightly  in 
staining  from  the  surrounding  mesenchyma  that  it  cannot  be  well  made  out 
without  the  use  of  a  higher  magnifying  power.  It  is  enclosed  by  a  distinct  mem 
brane  which  is  thick  enough  to  produce  a  double  outline,  and  contains  a  consid- 
erable number  of  scattered  nuclei,  which  are,  however,  nowhere  much  crowded. 
The  nuclei  are  round  in  form,  decidedly  larger  than  those  of  the  surrounding 
mesenchyma,  granular,  and  containing  each  several  more  conspicuous,  darkly 
staining  granules.  There  is  a  very  slight  gathering  of  mesenchymal  cells  about 
the  notochord,  as  if  to  form  the  anlage  of  a  sheath.  Just  Jpelow  the  notochord 
there  is  a  broad  band  of  somewhat  darker  staining,  due  to  a  greater  condensation 
of  the  mesenchyma  in  that  region,  and  this  condensation  represents  the  beginning 
of  the  formation  of  the  vertebral  structures.  On  either  side  we  find  the  trans- 


188 


STUDY  OF  PIG  EMBRYOS. 

EC.  Sp.c. 


Afr. 


-    01J. 


/'.  M. 


I..  V. 


FIG.  117. — PIG,  12.0  MM.    TRANSVERSE  SERIES  5,  SECTION  292. 

Ao.D,  Descending  aorta.  car. in,  Internal  carotid  artery.  cl.H,  Second  gill  cleft.  £c,  Ectoderm.  ff, 
Anlage  of  cerebral  hemispheres.  Jtig,  Jugular  vein.  L.gr,  Lachrymal  groove.  L.V,  Lateral  ventricle 
of  brain.  Mdb,  Mandibular  arch.  MX,  Maxillary  process.  My,  Myotome.  ncA,  Notochord.  N.cerv.i, 
First  cervical  nerve.  ^.7,  Facial  nerve.  N.q,  Glossopharyngeal  nerve.  N.io,  Vagus  nerve.  N.  //, 
Spinal  accessory  nerve.  O.F,  Oral  fissure  or  space  between  the  head  and  mandibles.  Olf,  Olfactory  pit. 
P/i,  Pharynx.  P.  M,  Pia  mater.  Sp.c,  Spinal  cord.  X  22  diams. 


TRANSVERSE  SECTIONS  OF  EMBRYO   OF  12  MM.  189 

formed  myotome,  My,  or  anlage  of  the  striated  muscular  tissue.  This  tissue  is 
produced  from  the  secondary  segments  of  earlier  stages.  The  cells  have  now  sepa- 
rated from  one  another,  have  lost  their  distinctly  segmental  grouping,  and  have 
begun  to  elongate  into  true  muscle-fibers.  All  that  can  be  distinguished  with 
the  low  power  is  the  somewhat  darker  appearance  of  this  part  of  the  section,  due 
to  the  great  crowding  of  the  nuclei.  Between  the  muscular  anlage  and  the  noto- 
chord  the  section  shows  a  portion  of  the  first  cervical  nerve,  N.  cerv.  i,  and  just 
within  this  nerve  is  a  small  blood-vessel  not  represented  in  the  figure.  There  is 
a  similar  blood-vessel  symmetrically  placed  on  the  opposite  side.  They  are  the 
small  vertebral  arteries.  The  jugular  or  anterior  cardinal  veins,  Jug,  are  large 
and  conspicuous  vessels,  but  despite  their  size  they  have  merely  endothelial  walls 
and  there  is  no  condensation  of  the  mesenchymal  cells  around  them,  although 
such  a  condensation  is  to  take  place  later  to  form  the  anlages  of  the  muscular 
and  connective-tissue  coats  (media  and  adventitia)  of  the  adult.  On  the  dor- 
sal side  of  the  jugular  vein  and  close  to  it  is.  a  light  spot  in  which  can  be  easily 
distinguished,  with  the  high  power,  several  more  or  less  distinct  bundles  of  nerve- 
fibers  which  are  separated  from  one  another  by  mesenchymal  cells.  For  this 
reason  it  is  somewhat  difficult  to  recognize  this  nerve  with  the  low  power  or  to 
represent  it  in  the  figure.  On  the  ventral  side  of  the  vein  there  appears  a  darkly 
stained  mass,  N.  10,  the  nodosal  ganglion  of  the  vagus  nerve,  and  outside  of  this 
ganglion  is  the  section  of  the  spinal  accessory  nerve.  Immediately  below  the  tiodo- 
sal  ganglion  we  have  the  internal  carotid  artery,  car.  in.  A  little  to  the  inside  of 
the  jugular  is  a  small  vessel,  Ao.D,  of  great  morphological  importance.  The  cor- 
responding vessel  appears  on  the  opposite  side.  Although  very  small,  this  vessel 
has  a  distinct  coat  of  condensed  mesenchyma  around  its  endothelium.  The  two 
vessels  are  the  descending  aortae,  which  have  almost  completely  aborted,  and  in 
slightly  older  specimens  will  be  found  to  have  disappeared  altogether.  The 
descending  aortae  are  the  longitudinal  trunks  by  which  the  dorsal  ends  of  the  five 
aortic  arches  of  early  stages  are  connected  together.  The  portion  showrn  in  this 
section  is  the  part  of  the  descending  aorta  between  the  tops  of  the  third  and 
fourth  aortic  arches.  The  relations  are  shown  in  the  reconstruction  (Fig.  97). 
The  pharynx,  Ph,  is  narrow  in  its  dorsal  ventral  diameter,  but  wide  transversely, 
and  offers  the  very  characteristic  yoke-shaped  figure  in  the  section.  The  distal 
portions  of  the  second  gill  clefts  are  shown,  and  they  appear  disconnected  with 
the  pharynx,  the  connection  occurring  in  sections  higher  up.  Each  cleft  is  some- 
what slit-like,  so  that  its  cavity  is  an  oblique  fissure  and  somewhat  parallel  in 
position  to  the  first  cleft  (Fig.  116).  Both  the  pharynx  and  the  gill  clefts  are, 
of  course,  lined  throughout  by  entoderm,  which  forms  a  sharply  defined  layer 
crowded  everywhere  with  nuclei, which  are  of  about  the  same  size  as  those  of  the 


190  STUDY  OF  PIG  EMBRYOS. 

surrounding  mesenchyma.  In  the  pharynx  the  entoderm  is  somewhat  thinner 
on  the  dorsal  than  on  the  ventral  side.  In  the  clefts  it  is  thicker  than  in  the 
pharynx  proper,  and  especially  in  the  clefts  it  may  be  observed  that  the  mitotic 
figures  always  occupy  a  superficial  position.  On  the  dorsal  side  of  the  cleft  is  a 
very  small  blood-vessel,  near  which,  with  a  higher  power,  one  may  see  a  small 
nerve,  and  nearby,  but  more  dorsal  wards,  a  second  nerve.  Both  of  these  are 
branches  of  the  glosso-pharyngeus,  and  lie  behind  the  cleft.  They  are,  there- 
fore, termed  the  post-trematic  branches.  Below  the  cleft  and  somewhat  on  its 
median  side  is  a  similar  third  nerve-branch,  the  pretrematic  of  the  glosso- 
pharyngeus,  running  in  front  of  the  cleft.  The  outline  of  the  embryo  forms  a 
rounded  eminence  outside  of  the  second  cleft ;  it  represents  in  part  the  hyoid  arch. 
In  the  midst  of  the  mesoderm  of  this  appears  a  light  area  with  a  few  nerve-fibers, 
the  end  of  the  facial  nerve,  N.  7.  The  mandibular  arch  or  process,  Mdb,  is  very 
distinct  and  prominent.  It  is  separated  from  the  hyoid  arch  by  a  deep  external 
notch,  which  corresponds  to  the  external  first  or  auditory  cleft.  In  the  interior 
of  the  mandibular  process  there  are  light  spaces  differing  in  their  exact  distribu- 
tion on  the  two  sides  of  the  specimen.  These  spaces  contain  nerve-fibers  and 
they  represent  the  inferior  maxillary  nerve.  We  now  come  to  the  oral  fissure, 
O.  F,  which  separates  the  body  from  the  head.  In  the  head  portion  of  the  sec- 
tion we  have  the  maxillary  process,  MX,  which  is  separated  in  part  from  the  rest 
of  the  head  by  the  deep  lachrymal  groove,  L.  gr.  On  either  side  there  shows  a 
shaving  from  the  epithelium  of  the  olfactory  chamber,  Olf.  The  fore-brain  has 
expanded  laterally,  L.  V,  to  form  the  lateral  ventricles,  the  walls  of  which,  H,  are 
the  anlages  of  the  cerebral  hemispheres.  On  the  dorsal  side,  which  is  the  lower 
side  in  the  figure,  the  hemispheres  project  somewhat,  leaving  a  median  space  be- 
tween them.  This  median  space  is  filled  with  mesenchyma,  which  may  already 
be  regarded  as  the  anlage  of  the  falx.  In  the  tissue  of  the  falx  are  two  very 
small  blood-vessels,  the  forward  prolongations  of  the  lateral  jugulars,  which  are 
to  unite  to  form  the  median  superior  longitudinal  sinus.  In  the  previous  section 
these  vessels  also  reappear,  but  are  already  united  (Fig.  116).  In  the  median 
dorsal  line  the  wall  of  the  fore-brain  is  thin  and  shows  a  characteristic  notch. 
Close  to  the  surface  of  the  fore-brain  there  is  a  very  distinctly  marked  vascular 
layer,  the  commencing  pia  mater,  P.  M,  and  with  a  high  power  it  can  be  easily 
seen  that  the  differentiation  of  the  arachnoid  zone  has  already  begun. 

Section  through  the  Third  Gill  Cleft  and  Nasal  Pits. — In  this  section  the  head 
is  clearly  separated  by  a  considerable  space  from  the  rest  of  the  section.  The 
transverse  diameter  of  the  embryo  is  here  much  less  than  higher  or  lower, 
so  that  the  section  as  a  whole  seems  somewhat  narrow.  It  shows  the  entire 
length  of  the  third  gill  cleft,  cl.  Ill,  exhibiting,  on  one  hand,  its  connection  with 


TRANSVERSE  SECTIONS  OF  EMBRYO  OF  12  MM.  191 

the  median  pharynx,  and,  on  the  other  hand,  its  dorsal  extremity,  where  its  ento- 
derm  joins  the  ectoderm.  The  external  outline  of  the  embryo  makes  a  deep 
depression  outside  the  end  of  the  third  cleft.  This  depression  is  the  cervical 
sinus  (compare  Fig.  96,  C.  S;  pig  of  10  mm.).  In  the  section  the  cervical  sinus 
displays  a  narrow  downward  prolongation.  If  followed  through  in  the  series  of 
sections,  this  prolongation,  which  is  on  the  inside  of  the  hyoid  arch,  Hy,  will  be 
found  to  connect  with  the  second  cleft.  The  spinal  cord,  Sp.  c,  presents  essen- 
tially the  same  structure  as  in  figures  116  and  117.  Our  section  passes  through 
the  roots  of  the  second  cervical  nerve,  N.cerv.  2,  and  shows  both  the  dorsal  gan- 
glion and  the  ventral  root  arising  from  the  ventral  zone.  These  two  roots  join 
and  form  the  nerve-trunk,  Nv.  2,  which  almost  immediately  divides,  sending  one 
branch  vertically  upward  into  a  mass  of  denser  crowded  cells  (the  anlage  of  the 
dorsal  musculature)  and  a  ventral  branch  which  descends  almost  vertically  to- 
ward the  pharynx.  Just  inside  of  this  ventral  brancj/we  have  the  section  of  the 
vertebral  artery,  Art.  -v.  Between  the  dorsal  suminit  of  the  ganglion  and  the 
spinal  cord  there  is  a  minute  bundle  of  nerve-^bers  not  shown  in  the  figure. 
These  fibers  constitute  the  commissural  trunk  of  the  eleventh  nerve.  The  third 
gill  cleft  is  cut  almost  symmetrically,  and  extends  from  the  median  line  to  the 
edge  of  the  section.  It  is  lined  throughout  by  the  entoderm,  which  at  the  end 
01  the  cleft  on  each  side  has  met  and  fused  with  the  ectoderm  to  form  the  epithe- 
lial membrane,  the  closing  plate.  The  membrane  apparently  normally  remains 
intact  in  mammals.  In  the  ichthyopsida  the  membrane  becomes  ruptured  dur- 
ing embryonic  life,  and  the  gill  cleft  is  opened  to  the  exterior.  At  the  end  of  the 
left  the  entoderm  has  undergone  a  special  growth  forming  a  distinct  mass,  Thm, 
>n  the  side  of  the  cleft  toward  the  head.  This  entodermal  structure  is  the  anlage 
•f  the  thyrnus  gland  and  is  already  penetrated  by  small  blood-^  '^Hi  art 

lerhaps  not  capillaries,  but  sinusoids.     The  student  should  clearly  understand 
hat  the  median  region  of  the  third  gill  cleft  is  really  the  pharynx  proper.     From 
ts  median  ventral  line  arises  the  beginning  of  the  trachea,  7Y,  which  should,  per- 
iaps,  be  already  designated  as  the  anlage  of  the  larynx.     The  entoderm  extends 
[own  in  the  median  line  for  a  considerable  distance,  making  a  figure  which,  in 
i  he  section,  is  shaped  somewhat  like  a  spear-head.     In  the  center  o*  the  section 
appears  a  small  cavity.     Further  down  toward  the  lungs  we  have  o*  V  an  epi- 
thelial   plate  with  no  cavity  observable  in  it  (Fig.   119,   Tra),  the  entcHcrm 
-of  the  trachea  at  this  stage  forming  a  solid  cord.     Ventrad  from  the  trache*., 
in  the  median  region  and  between  the  two  aortic  arches,  is  a  small,  irregular, 
deeply  stained  mass  01'  cel1^ .Thyr,  the  anlage  of  the  thyroid  gland.     These 
cells  are  entodermal,  the  an  .          |$ring  been  developed  by  a  downgrowth  of  the 
pithelium  of   the  floor  of  pharynx,  although  at  the  present  stage  the 


192 


STUDY  OF  PIG  EMBRYOS. 


original  connection  with  the  pharynx  has  been  lost.     The  anlage  is  now  isolated 
from  its  parent  germ-layer  and  is  imbedded  in  mesenchyma.     It  is  solid,  for 


Ol.pl. 


FIG.  118. — PIG.  12.0  MM.     TRANSVERSE  SERIES  5,  SECTION  321. 

• 

Au.4,    Four   i        tic  arch.     Art. v,  Vertebral  artery.     Au,    External  auditory  .cleft,     cl.ni,  Third  internal  gill 

cleft.      ' ~  nod,  Ganglion  nodosum.     Hy,  Hy old  arch.     Jug,  Jugular  vein,      L.  V,  Lateral  ventricle.     Na, 

N.cerv.i,  First  cervical  nerve.      N.cerv.2,   Second  cervical  nerve.      AT.if,  Spinal  accessory 

-  Hypoglossal  nerve.      Nv.2,  Main  trunk  of  second  cervical  nerve.       Ol.pl,  Olfactory  plate. 

.0,   Ramus  externus  of  the  glossopharyngeus.      Sf.c,  Spinal  cord.       T/im,  Thymus.      T/tyr,  Thtbid. 

Tr,  Trachea.      X  22  diams. 

the  cavities  of  the  thyroid  follicles  are  not  develops?  until  considerably  later. 
Just  above  the  third  gill   cleft   may  be  seen  a    '  rge,  d:  .ained  mass, 


TRANSVERSE  SECTIONS  OF  EMBRYO  OF  12  MM.  193 

G.  nod,  the  ganglion  nodosum  of  the  vagus  nerve.  Immediately  above  it  is 
the  section  of  the  jugular  or  anterior  cardinal  vein,  Jug.  Betweernthe  ganglion 
and  the  vein  is  a  bundle  of  nerve-fibers  representing^the  twelfth  or  hypo- 
glossal  nerve.  Close  to  the  ganglion  on  its  outer  side  is  the  section  of  the 
spinal  accessory  nerve,  N.  H,  which  reappears  again  in  the  section  below  the 
pharynx,  at  N.  12.  The  reason  for  this  double  appearance  of  the  hypoglos- 
sal  nerve  may  be  seen  readily  by  examination  of  the  reconstruction  (Fig.  105). 
A  little  above  the  jugular  vein  is  the  stction  of  the  first  cervical  nerve,  N.  eery,  i, 
laterad  from  which  is  the  external  branch,  R.ex  p,  of  the  spinal  accessory  nerve. 
This  branch  in  the  adult  innervates  the  sternocleidomastoid  and  trapezius 


The  lower  part  of  the  figure  represents  the  section  of  the  head  and  shows 
the  two  nasal  fossae,  Na,  closed  toward  the  mouth  side  by  the  olfactory  plate, 
/'/,  the  epithelial  membrane  somewhat  resembling  the  closing  plate  of  a  gill 
cleft,  but  it  is  formed  by  a  fusion  of  the  ectoderm  on  the  two  sides  of  the  opening 
of  the  nasal  pits.    When  the  nasal  pits  are  first  formed,  they  are  open  throughout 
their  whole  extent.     The  formation  of  the  olfactory  plate  is  the  first  step  toward 
the  separation  of  the  two  nasal  cavities  from  the  oral  cavity.     In  later  stages  this 
plate  disappears,  and  its  forward  portion  is  replaced  by  mesenchyma,  so  that  the'- 
separation  of  the  nasal  and  oral  cavities  is  permanent,  but  the  posterior  portion 
of  the  membrane  becomes  first  very  thin,  and  finally  disappears  altogether,  thus 
^  ablishjng  a  secondary  connection  between  the  nose  and  mouth  for  each  nasal 
,  imber,  thus  leading  to  the  development  of  the  internal  choanae.     On  the  dorsal 
e  of  the  nasal  pits  (below  in  Fig.  118),  the  cerebral  hemispheres  are  cut  sepa- 
rately, their  darkly  stained  walls  bounding  on  each  side  the  large  lateral  ventricle. 
Section   through   the  Fourth  Gill  Cleft.  —  Of   the   entodermal   gill  clefts  or 
pouches  the  fourth  is  by  far  the  smallest,  and  as  it  appears  in  seci  <  .g.  T  19, 

cl.  IV)  is  inconspicuous.  The  section  figured  differs  byiv  o  striking  features 
from  those  of  the  series  above  described:  -first,  because  the  head  is  no  longer 
included;  and,  second,  because  the  cardiac  structures  are  beginning  to  show. 
On  the  dorsal  side  the  spinal  cord  is  cut  at  the  level  of  the  ganglion,  G.-j,  of  the 
third  cervical  cleft.  The  dorsal  root  of  the  ganglion  joining  the  spinal  cord,  Sp.  ' 
Q,  is  shown  on  both  sides  of  the  section,  and  the  nerve  itself  also  appears,  being 
best  shown  on  the  left  side  of  the  embryo,  where  a  short  piece,  R.  D.  j,  of  the 
ramus  dorsalis  is  included  and  a  much  longer  piece,  R.  V.  j,  of  the  ramus  ven- 
tralis.  Just  inside  of  the  nerve  at  the  level  of  the  notochord,  Nch,  is  the  cross- 
section  of  the  vertebral  artery.  On  the  right  side  of  the  embryo  the  section 
.passes  through  a  portion  of  the  second  cervical  nerve,  N.  cerv.  2.  The  jugular 
vein,  Jug,  is  a  very  large  vessel.  Close  t^  its  ventral  wall  appear  a  few  fibers 


194 


STUDY  OF  PIG  EMBRYOS. 


which  represent  the  first  cervical  nerve,  but  they  are  too  indistinct  to  be  repre- 
sented in  the  figure.  They  may  easily  be  found  with  the  higher  power.  In  the 
median  plane  is  the  crescent-shaped  section  of  the  pharynx,  Ph.  Between  the 
jugular  vein  and  the  pharynx  lie?  the  fourth  aortic  arch,  Ao.  4.  The  right  and 
left  arches  are  at  this  stage  about  equal  in  size,  although  the  left  arch  is  Destined 


Sp.c. 


Cce. 


x  -Pic,  12. o  MM.     TRAXSVERSE  SERIES  5,  SECTION  353. 

Ao,    (\orta.     Ao.j,    Fourth  a  Right  auricle.      Au.S,   Left  auricle.       Ca',   Ccelom.      <-/.//. 

Fourth   gill    pouch.      6".,,  Gai.fo  '  ervical    nerve.     Jug,    Jugular    vein,      «/.<///,   Mesothelium. 

Ar.cen>.2,  Second  cervical  nerve.  /Vc/i,  iNoi.OL.iord.  JVr.  io,ir,  United  vagus  and  spinal  accessory  nerves. 
P.A,  Pulmonary  artery.  Ph,  Pharynx.  R.D.  j,  Dorsal  ramus  of  the  third  cervical  nerve.  R.  F.j, 
Ventral  ramus  of  the  third  cervical  nerve.  Som,  Somatopleure.  S/>.c,  Spinal  cord.  Syrn,  Sympathetic 
nerve  chain.  Trot,  Trachea.  Ft,  Vein  to  lower  jaw.  X  22  diams. 


to  form  the  main  aortic  arch  of  the  body,  and  only  a  portion  of  the  right  arch 
will  persist  to  form  a  portion  of  the  stem  of  the  pulmonary  artery.  The  figure 
indicates  the  manner  in  which  these  aortic  arches  pass  up  from  the  heart  below 
on  either  side  of  the  pharynx.  A  little  above  the  aortic  arch  on  either  side  may 
n  a  small,  round  spot,  Sym,  winch  is  somewhat  conspicuous  on  account  of 


TRANSVERSE  SECTIONS  OF  EMBRYO  OF  12  MM.  195 

its  deeper  staining^.  It  is  a  section  of  the  cervical  sympathetic.  Examination 
with  a  higher  power  shows  that  it  consists  of  somewhat  crowded  cells,  some  of 
which  have  larger  nuclei.  These  are  the  neuroblasts.  The  mesenchymal  cells 
immediately  around  the  anlage  are  disposed  about  it  in  somewhat  concentric 
lines.  Between  the  jugular  and  the  aortic  arch  is  situated  the  large  conspicuous 
nerve-trunk,  N.  10,  //,  constituted  by  the  united  vagus  and  spinal  accessory 
nerves.  Below  this  double  nerve  is  a  blood-vessel,  Ve,  a  branch  of  the  jugu- 
lar vein  which  runs  to  the  lower  jaw  and  tongue.  The  homologies  between  this 
vein  and  those  of  the  adult  have  not  yet  been  worked  out.  Returning  now  to 
the  pharynx,  Ph*-  on  the  right  side  the  prolongation  of  the  pharynx  to  join  the 
fourth  cleft  can  be  clearly  followed ;  on  the  left  side  of  the  embryo,  the  right  of 
the  figure,  the  fourth  cleft,  cl.  IV,  does  not  display  its  connection  with  the 
pharynx,  but  is  a  separate,  small  epithelial  cavity  lined  by  a  cylinder  epithelium. 
Underneath  the  pharynx  appears  a  vertical  plate,  Tra,  formed  by  the  entoderm 
of  the  trachea.  This  plate  is  thinnest  in  the  middle,  somewhat  wider  toward 
the  top  and  bottom  of  the  section.  It  is  quite  solid,  except  for  a  minute  cavity 
at  its  dorsal  end.  This  minute  cavity  may  be  traced  from  the  opening  of  the 
glottis  through  the  series  of  sections  down  until  it  becomes  connected  with  the 
comparatively  large  cavities  of  the  developing  bronchi  of  the  lung.  Below  the 
pharyngeal  region  descends  the  thick  somatopleure,  Som,  which  encloses  the 
pericardial  coelom,  Cos,  in  which  the  heart  is  lodged.  The  inner  surface  of  the 
somatopleure  is  covered  by  the  thin  mesothelium,  msth.  Of  the  cardiac  struc- 
tures we  note  first  the  section  of  the  main  aorta,  Ao,  and  of  the  pulmonary  aorta, 
P.  A,  and  finally  small  sections  of  the  uppermost  part  of  the  two  auricles,  Au.  d 
and  Au.  s.  More  of  the  left  auricle  is  included  in  the  section  than  of  the  right. 

Section  through  the  Anterior  Limbs  and  Heart. — The  section  figured  is  much 
lower  in  the  series  than  the  last  and  was  selected  in  order  to  illustrate  the  ante- 
rior limb-buds,  the  ducts  of  Cuvier,  and  the  heart.  The  position  and  shape  of 
the  limb-buds  are  sufficiently  shown  in  figure  96.  The  section  demonstrates ' 
that  the  limb-bud  is  formed  chiefly  by  a  dense  mass  of  mesoderm  covered  by  a 
thin  layer  of  ectoderm.  The  mesoderm  consists  of  very  much  crowded  cells  in 
which  it  is  very  difficult  to  recognize  any  distinct  differentiations,  yet  it  is  prob- 
able that  here  are  mingled  both  true  mesenchymal  cells  and  cells  which  originally 
belonged  to  the  muscle  plates,  but  which  have  now  broken  apart  and  are  devel- 
oping singly  into  muscleTfibers.  In  certain  amphibia  the  cells  from  the  muscle 
plate  can  be  distinguished  from  the  mesenchymal  cells  of  the  limb,  and  what  we 
know  of  the  development  of  the  muscles  in  amniota  confirms  tr,e  view  that  stri- 
ated muscles  and  mesenchyma  are  genetically  entirely  distinct.  No  skeletal 
elements  whatever  have  yet  arisen  in  the  limb.  We  have  here  a  striking  illus- 


196  STUDY  OF  PIG  EMBRYOS. 

tration  of  the  fact  that  the  skeleton  is  very  late  in  its  development,  and,  embryo- 
logically  speaking,  is  in  no  sense  the  framework  upon  which  the  body  is  built  up, 
but  rather  a  late  supplementary  development.  The  main  morphological  fea- 
tures in  all  parts  of  the  embryo  are  entirely  fixed  by  the  soft  tissues  before  the 
skeletal  structures  arise.  Both  nerves  and  blood-vessels  have  grown  into  the 
limb.  The  nerves  are  the  ventral  branches  of  the  spinal  nerves.  Several  of 
these  unite  together  and  form  the  brachial  plexus,  one  part  of  which,  BY.  plx,  is 
shown  in  the  section.  In  the  present  embryo  this  nerve-trunk  includes  fibers 
derived  from  both  the  sixth  and  seventh  cervical  nerves.  Just  above  the  nerve- 
trunk  is  the  section  of  the  subclavian  or  axillary  vein,  which  is  a  branch  from  the 
jugular.  The  dorsal  region  of  the  embryo  is  relatively  larger  at  the  level  of  this 
section  than  higher  up,  owing  chiefly  to  the  great  development  of  the  mesoderm. 
The  spinal  cord,  Sp.  c,  resembles  that  in  figure  119,  but  is  both  larger  and  more 
differentiated.  On  the  left  side  of  the  embryo  the  fundamental  morphological 
characteristics  of  the  spinal  nerve  are  well  illustrated  in  this  section.  The  dorsal 
root,  D.  R,  bears  the  ganglion,  G,  which  joins  the  dorsal  zone  of  the  spinal  cord. 
The. fibers  of  this  root  are  produced  from  the  cells  of  the  ganglion  and  grow  from 
the  ganglion  into  the  spinal  cord.  Other  fibers  from  the  same  cells  growr  out  in 
the  opposite  direction  and  form  the  nerve-trunk  or  root  which  descends  from  the 
ganglion  in  a  nearly  vertical  direction.  The  ventral  root,  V.  R,  arises  from  the 
ventral  zone,  takes  an  oblique  course,  and  joins  the  dorsal  root  a  little  below  the 
level  of  the  spinal  cord  to  form  a  single  nerve-trunk,  which,  however,  soon  sub- 
divides into  its  two  primary  branches.  The  first  or  dorsal  branch,  R.D,  bends  at 
an  acute  angle  upward  and  outward.  The  second  or  ventral  branch,  ramus 
ventralis,  continues  downward  and  curves  into  the  limb.  Owing  to  this  curva- 
ture, in  order  to  follow  its  course  the  nerve  must  be  traced  through  adjacent 
sections.  If  this  is  done,  the  ventral  ramus  will  be  found  to  take  part  in  the 
formation  of  the  brachial  plexus.  Some  distance  below  the  spinal  cord  is  the 
small  notochord.  Further  down,  but  also  in  the  median  line,  appear  two  small 
rings  of  epithelium.  Of  these,  the  smaller  upper  one,  (E,  is  the  entodermal  lining 
of  the  oesophagus,  and  the  larger  lower  one  is  the  entodermal  lining  of  the  trachea. 
Around  each  of  these  rings  there  has  already  occurred  a  slight  condensation  of 
the  mesenchyma,  the  first  step  toward  the  ultimate  differentiation  of  the  sub- 
mucous  and  muscular  coats  of  the  oesophagus  and  trachea.  The  entoderm  of 
both  the  oesophagus  and  trachea  is  a  moderately  thick  layer  composed  of  elon- 
gated cells,  the  nuclei  of  which  are  distributed  at  various  levels,  but  so  as  to 
leave  the  superficial  portion  of  the  layer  comparatively  free.  It  is  in  this  super- 
ficial portion  that  the  mitotic  figures  always  occi^.  On  the  ventral  side  of  the 
trachea  and  quite  close  to  it  appear  two  small  blood-vessels,  the  pulmonary  arte- 


TRANSVERSE  SECTIONS  OF  EMBRYO  OF  12  MM. 


197 


ries.     By  an  oversight  the  two  arteries  are  represented  as  one  in  the  engraving. 
To  the  right  and  left  of  the  oesophagus  appear  the  circular  sections  of  the  two 


Sp.c. 


D.R, 


Nch. 


Som. 


FIG.  120.— PIG,  12. o  MM.     TRANSVERSE  SERIES  5,  SECTION  470. 

Ao.S,  Left  descending  aorta.  Au.d,  Right  auricle.  Br.Plx,  Brachial  plexus.  D.C.S,  Ductus  Cuvieri 
sinistra.  D.R,  Dorsal  root  of  spinal  nerve.  F,  Cardiac  fissure.  G,  Spinal  ganglion.  Z,  Anterior  limb-bud. 
Nch,  Notochord.  Nv,  Branch  of  brachial  plexus.  (E,  (Esophagus.  R.D,  Ramus  dorsalis  of  spinal 
nerve.  S.a.c,  Septum  of  the  auricular  canal.  Scl.V,  Subclavian  vein.  Som,  Somatopleure.  Sp.c,  Spinal 
cord.  S.s,  Septum  superius.  Tra,  Trachea.  Val,  Auriculo-ventricular  valve.  Ven.S,  Left  ventricle  of 
the  heart.  V.R,  Ventral  root  of"spinal  nerve.  X  22  diams. 

descending  aortae,  of  which  ttfl'left,  Ao.  S,  is  already  somewhat  larger  than  the 
right.     Ultimately  the  greater  part  of  the  right  aorta  will  disappear,  the  aortic 


198  STUDY  OF  PIG  EMBRYOS. 

arch  of  the  adult  being  formed  from  the  left  aorta.  Lower  down  in  the  series 
the  two  descending  aortse  unite  to  form  the  single  median  dorsal  aorta.  The 
ducts  of  Cuvier  are  two  enormous  venous  trunks  which  deliver  the  blood  to  the 
heart.  They  lie  symmetrically  placed  to  the  right  and  left  of  the  oesophagus 
and  trachea.  They  extend  from  the  level  of  the  descending  aortae  downward  and 
inward  to  the  level  of  the  heart.  The  duct  of  the  left  side,  D.  C.  S,  is  almost  sym- 
metrical with  its  fellow  of  the  right  side,  but  it  has  no  direct  communication  with 
the  heart ;  but  by  following  down  through  the  series  of  sections  the  student  can 
observe  that  the  left  duct  of  Cuvier  connects  across  with  the  duct  of  the  right 
side.  The  right  duct  opens  directly  into  the  right  auricle,  Au.  d,  of  the  heart. 
All  of  the  venous  blood  is  collected  at  this  stage  by  the  ducts  of  Cuvier,  except 
that  which  comes  through  the  liver.  The  dorsal  end  of  each  duct  of  Cuvier  is 
formed  by  the  union  of  the  jugular  or  anterior  cardinal  vein  from  the  head  with 
the  posterior  cardinal  vein  from  the  body.  The  opening  of  the  right  duct  into 
the  auricle  of  the  heart  is  guarded  by  two  small  flaps  or  valves.  The  lower  part 
of  the  section  is  occupied  by  the  large  heart  lying  in  the  pericardial  chamber. 
The  body-wall,  Som,  or  somatopleure,  which  forms  the  outer  covering  of  this 
chamber',  is  quite  thin  and  without  a  trace  of  muscular  or  skeletal  structures.  It 
consists  of  three  distinct  layers — an  external  ectoderm  and  middle  mesenchyma, 
and  the  internal  mesothelium.  The  mesothelium  is  a  thin  layer  of  cells  which 
persists  throughout  life  and  is  known  in  the  adult  as  the  pericardial  epithelium. 
In  the  present  section  it  is  easy  to  follow  this  layer  from  the  somatopleure  past 
the  ducts  of  Cuvier  on  to  the  heart  and  completely  around  the  outside  of  the 
heart  itself.  Everywhere  it  forms  the  covering  or  boundary  of  the  coelom  of  the 
pericardium.  In  later  stages  this  mesothelium  will  have  an  especial  layer  of 
connective  tissue  close  under  it.  The  layer  of  connective  tissue,  together  with 
the  mesothelium,  constitutes  the  pericardial  membrane  of  descriptive  anatomy. 
The  essential  fundamental  relations  of  this  membrane  may,  therefore,  be  easily 
understood  from  the  present  section.  From  the  study  of  the  adult  conditions 
alone  it  is  extremely  difficult  for  the  student  to  grasp  these  relations.  The  heart 
is  a  very  large  organ.  It  consists  of  two  auricles  and  a  ventricle  with  two  limbs. 
The  auricles  have  thin  walls  and  are  separated  from  one  another  by  a  very  thin 
membrane,  the  septum  superius,  S.  s.  The  right  auricle,  Au.  d,  receives  upon  its 
dorsal  side  the  opening  of  the  right  vein  or  duct  of  Cuvier,  the  opening  being 
guarded  by  valves.  Of  these  valves,  the  one  toward  the  median  line  disappears, 
but  the  other,  toward  the  right  of  the  embryo,  persists  to  form  both  the  Eusta- 
chian  and  Thebesian  valves  of  the  adult.  As  stated  above,  the  left  duct  of 
Cuvier  delivers  its  blood  to  the  right  duct,  and  so  indirectly  to  the  heart.  The 
ventricles  of  the  heart  are  much  larger  than  the  auricles,  and  the  left  ventricular 


TRANSVERSE  SECTIONS  OF  EMBRYO  OF  12  MM.  199 

limb  or  future  left  ventricle,  Ven.  S,  is  already  larger  than  the  right  limb.  The 
external  groove,  F,  which  marks  the  boundary  between  the  two  auricles  is 
clearly  shown  by  the  section.  The  trabecular  structure  of  the  ventricles  is  well 
developed,  and  affords  a  diagnostic  path  by  which  the  ventricles,  if  they  are  cut, 
may  be  easily  recognized  in  sections.  The  development  of  the  trabeculae  corre- 
sponds to  the  formation  of  blood  sinusoids  of  the  heart.  The  trabeculae  consist 
of  young  muscle  cells,  and  each  bundle  of  cells  is  closely  invested  by  the  endo- 
thelium  of  the  heart.  The  blood  thus  circulates  freely  between  the  trabeculae, 
but  remains,  as  in  every  blood-channel,  separated  by  the  endothelium  from  the 
neighboring  tissue.  The  tissues  of  the  heart  are  thus  enabled  to  get  their  nour- 
ishment from  the  blood  circulating  through  the  organ.  The  sinusoidal  type  of 
circulation  which  we  here  encounter  appears  in  the  heart  of  all  vertebrate  em- 
bryos, and  is  the  permanent  form  of  circulation  in  the  frog.  In  mammals,  on 
the  other  hand,  although  the  sinusoidal  circulation  is  kept  throughout  life  and 
trie  ventricles  always  have  their  trabecular  structure,  yet  we  find,  in  addition,  the 
development  of  a  true  capillary  circulation  to  supplement  the  sinusoidal.  This 
capillary  circulation  is  supplied  by  the  coronary  arteries,  and  develops  compara- 
tively late.  Between  the  auricles  and  the  ventricles  the  heart  is  narrow.  This 
constricted  region  is  known  as  the  auricular  ca.nal.  A  broad  partition,  5.  a.  c, 
divides  the  cavity  of  the  auricular  canal  into  right  and  left  channels,  forming 
open  vessels  between  the  auricles  and  ventricles.  From  the  lower  edges  of  these 
channls  flaps  of  tissue  project  into  the  ventricles.  The  flaps  are  the  anlages  of 
the  atrio- ventricular  valves. 

Sections  through  the  Anterior  Limbs  to  Show  the  Brachial  Plexus.^ Figure 
121  was  drawn  from  a  single  section,  except  that  the  nerves  in  the  limbs  repre- 
sent a  reconstruction  from  several  adjacent  sections.  The  limb-bud,  A.  L,  pro- 
jects freely  from  the  side  of  the  body,  is  covered  by  ectoderm,  EC,  and  filled  with 
a  very  dense  tissue,  the  cells  of  which  show  no  very  clear  histological  differentia- 
tion. The  spinal  cord,  Sp.  c,  is  fairly  well  advanced  in  its  development  at  this 
level,  and  shows  a  darker,  inner  layer,  Epen,  a  middle,  gray  layer,  cin,  and  an 
outer  neuroglia,  EC.  gl.  The  cord  is  completely  surrounded  by  the  developing 
pia  mater,  which  is  quite  thin,  but  highly  vascular.  The  ganglia  are  cut  almost 
symmetrically  on  the  two  sides  and  show  their  dorsal  roots.  The  descending 
trunk  from  each  ganglion  is  joined  by  the  ventral  roots,  V.  R,  which  arise  from 
the  ventral  zone  of  the  cord  in  several  bundles  which  unite  about  the  same  time 
with  both  one  another  and  the  dorsal  root  to  form  the  main  nerve-trunk,  N.S, 
which  enters  into  the  formation  of  the  brachial  plexus.  Just  after  the  junction 
of  the  two  roots  the  nerve  gives  off  a  branch  which  runs  obliquely  dorsalwards 
into  the  anlage  of  the  dorsal  muscles,  Muse.  This  branch  is,  of  course,  the  dorsal 


200 


STUDY  OF  PIG  EMBRYOS. 


TRANSVERSE  SECTIONS  OF  EJ^BRYO   OF  12  MM.  201 

ramus.  The  trunk,  N,  which  runs  toward  the  limb  is  the  ventral  ramus.  Below 
the  spinal  cord  is  the  notochord,  Nch,  which  is  completely  surrounded  by  a  very 
dense  mass  of  mesenchymal  cells,  Vert,  the  anlage  of  the  body  of  a  vertebra- 
Lower  down  in  the  section  are  the  two  descending  aortae,  Ao,  which  are  at  this 
point  just  uniting  to  form  the  single  median  dorsal  aorta.  Below  the  aorta  runs  a 
ring  of  epithelium,  (E,  representing  the  entoderm  of  the  oesophagus,  and  further 
ventralwards  a  second  layer  of  epithelium,  Tra,  the  entodermal  lining  of  the 
trachea.  Both  of  these  rings  of  epithelium  are  surrounded  by  somewhat  con- 
densed mesenchyma,  the  differentiation  of  which  about  the  oesophagus  is  more 
advanced  than  about  the  trachea.  Around  the  oesophagus  next  to  the  epithe- 
lium is  a  thin,  looser  layer  of  mesenchyma,  the  anlage  of  the  mesodermic  portion 
of  the  future  mucous  membrane,  and  perhaps  also  of  the  submucosa.  Outside  of 
this  looser  mesenchymal  envelope  is  a  second  denser  layer  in  which  the  cells  ap- 
pear elongated,  having  begun  their  differentiation  into  smooth  muscle  cells.  To 
the  right  and  the  left  of  the  aorta  appear  the  very  large  posterior  cardinal  veins, 
card.  From  the  sides  of  the  trachea  project  lobes  of  tissue  which  represent  the 
anlages  of  the  lungs.  These  lobes  of  tissue  are  each  covered  by  a  layer  of  meso- 
thelium,  and  protrude,  as  it  were,  into  the  coelom  of  the  pleural  cavities,  Pleur. 
Further  to  one  side  the  coslom,  Cos,  of  the  abdominal  cavity  is  also  in  part  shown. 
It  is  bounded  externally  by  the  body- wall,  Som,  of  the  embryo.  To  the  right  and 
the  left  of  the  oesophagus  lie  the  sections  of  the  vagus  nerve,  the  right  nerveJa 
little  higher  up  than  the  left.  Below  the  trachea  in  the  median  line  is  a  small 
blood-vessel,  a  section  of  the  pulmonary  vein.  Above  the  aorta,  on  the  right  and 
left,  lies  a  small  cluster  of  rather  darkly  stained  cells,  intermingled  with  which 
one  can  make  out  with  a  higher  power  the  future  nerve-fibers.  These  structures 
are  portions  of  the  sympathetic  nervous  system.  As  regards  the  great  nerve  of  the 
limb,  N.8,  it  must  be  remembered  that  it  forms  a  portion  of  the  brachial  plexus 
and  is  joined  by  other  cervical  nerves.  From  the  voluminous  trunk  thus  devel- 
oped there  arise  three  principal  branches;  the  first,  xx,  is  at  the  base  of  the  limb, 
is  small,  and  runs  off  dorsally.  The  other  two  represent  a  terminal  forking  of 
the  nerve- trunk,  one,  yy,  running  to  the  dorsal  side  of  the  limb,  the  other,  zz,  to 
the  ventral  side. 

Section  through  the  Stomach  and  Liver. — We  now  pass  to  a  section  well  below 
the  heart  in  order  to  study  the  characteristics  of  the  Wolffian  body,  stomach,  and 
liver.  At  this  level,  as  comparison  with  figures  1 18  and  120  will  show,  the  body 
of  the  embryo  has  its  greatest  dimensions.  The  upper  edge,  Urn,  of  the  umbilical 
cord  appears  in  this  section.  The  spinal  cord  with  its  ganglia  and  nerves  pre- 
sents essentially  the  same  features  as  in  figure  120.  The  notochord,  Nch, 
forms  a  small  circle  in  section  and  is  surrounded  by  the  anlage  of  the  body 


202  STUDY  OF  PIG  EMBRYOS. 

of  a  vertebra,  which  appears  as  an  area  relatively  large,  over  which  the  mesen- 
chymal  cells  are  more  crowded  or  condensed  than  elsewhere.  At  its  periphery  the 
anlage  merges  without  divisional  boundary  into  the  surrounding  mesenchyma. 
It  is  more  expanded  laterally  than  ventrally.  In  the  median  line  below  the 
notochord  is  the  large  dorsal  aorta,  Ao,  which  is  formed  by  the  union  of  the  two 
descending  aortae  shown  in  figure  120,  and  which  extends  through  the  abdominal 
region  of  the  embryo  to  the  pelvic  region,  where  it  forks  to  form  the  two  allantoic 
arteries,  which,  passing  on  either  side  of  the  intestine,  continue  their  course  along 
the  side  of  the  internal  allantois  or  future  bladder,  until  they  reach  the  umbilicus, 
where  they  enter  the  umbilical  cord  to  supply  the  extra-embryonic  or  placental 
circulation.  The  aorta  is  surrounded  by  mesenchyma,  and  to  this  are,  so  to 
speak,  appended  the  large  Wolffian  bodies,  W.  B,  one  on  each  side.  From  the 
dorsal  region  of  the  embryo  to  the  umbilical  cord  extends  the  somatopleure  or 
body-wall,  Som,  which,  like  that  around  the  pericardial  chamber,  consists  of  an 
external  ectoderm,  EC,  a  middle  mesenchyma,  mes,  and  an  internal  mesothelium, 
msth.  It  is  important  for  the  student  to  understand  the  arrangement  of  the 
germ-layers  in  the  somatopleure.  The  mesothelium  is  commonly  known  in  the 
descriptive  anatomy  of  the  adult  as  the  peritoneal  epithelium.  The  peritoneal 
membrane  consists  of  this  epithelium  and  of  all  underlying  connective  tissue.  In 
sections  like  that  figured  it  can  be  readily  followed  not  only  over  the  inner  surface 
of  the  body- wall,  but  over  the  surface  of  the  Wolffian  body  and  liver,  and  upon 
the  left  side  of  the  body  also  over  the  surfaces  of  the  greater  omentum,  stomach, 
and  lesser  omentum.  The  relations  of  the  abdominal  viscera  to  the  peritoneum 
which  are  so  perplexing  to  the  student  of  adult  anatomy  are  here  shown  diagram- 
matically,  as  it  were,  by  the  section  of  the  actual  embryo.  It  is  evident  from 
such  a  section  that  the  abdominal  cavity  (splanchnocele)  is  completely  bounded 
by  mesothelium,  and  that  all  the  abdominal  viscera  are  outside  of  the  cavity. 
This  conception,  which  is  so  important,  yet  so  difficult  to  the  student  of  anatomy, 
is  easily  mastered  by  the  study  of  embryonic  relations.  The  Wolffian  body, 
W '.  B,  is  the  foetal  or  embryonic  kidney,  and  is  also  termed  the  mesonephros  (com- 
pare page  10 1 ).  It  is  much  larger  relatively  to  other  parts  in  the  pig  than  in  man 
or  the  rabbit.  It  consists  of  numerous  epithelial  tubules  very  much  contorted 
with  blood  spaces  between  them,  of  glomeruli  which  always  lie  toward  the  median 
and  inferior  side  of  the  organ,  and,  finally,  of  a  single  longitudinal  canal,  the  Wolff- 
ian ducv,  into  which  all  of  the  tubules  open.  The  tubules  are  formed  by  the 
cuboidal  epithelium.  The  glomeruli  resemble  in  their  structure  those  of  the 
kidney.  Each  is  a  bunch  of  blood-vessels  covered  in  by  a  layer  of  epithelium 
which  forms  one  boundary  of  the  space  into  which  the  glomerulus  projects.  The 
opposite  side  of  the  i.pace  is  also  bounded  by  epithelium,  which  at  the  stalk  of  the 


TRANSVERSE  SECTIONS  OF  EMBRYO  OF  12  MM. 


203 


glomerulus  becomes  continuous  with  the  covering  of  the  glomerulus  itself,  the 
whole  structure  resembling  closely  that  of  a  Malpighian  corpuscle  of  the  true  kid- 
ney. The  space  around  each  glomerulus  is  really  the  beginning  of  a  Wolffian 


Nch. 


Vert. 


V.  U.  D. 


v.u.s. 


FIG.   122. — PIG,  12. o  MM.     TRANSVERSE  SERIES  5,  SECTION  633. 

Ao,  Dorsal  aorta.  EC,  Ectoderm.  G,  Spinal  ganglion.  G.bl,  Gall-bladder.  Gen,  Anlage  of  genital  gland. 
Li,  Liver,  tries,  Somatic  mesenchyma.  msth,  Somatic  mesotheliumi  ~N~,  Spinal  nerve.  Nch,  Notochord. 
Om.maj,  Omentum  majus.  Om.min,  Omentum  minus.  Soni,  Somatopleure.  Sp.c,  Spinal  cord.  St, 
Stomach.  Um,  Umbilical  cord.  V.card,  Posterior  cardinal  vein.  V.C.I,  Vena  cava  inferior.  Vert, 
Anlage  of  vertebra.  V.U.D,  Right  umbilical  vein.  V.U.S,  Left  umbilical  vein.  W.B,  Wolffian  body. 
X  22  diams. 

tubule.    The  spaces  between  the  tubules  are  almost  entirely  blood-channels,  and 
are  lined  by  endothelium,  which,  for  the  most  part,  is  closely  fitted  against  the 


204  STUDY  OF  PLG  EMBRYOS. 

epithelium  of  the  tubules.  Occasionally  a  small  amount  of  mesenchyma  can  be 
found  between  the  tubules,  or  between  the  tubules  and  the  nearest  endothelium. 
We  have,  accordingly,  in  these  organs  a  typical  sinusoidal  circulation.  The 
blood  spaces  of  the  Wolffian  body  really  belong  to  the  posterior  cardinal  veins 
into  which  the  Wolffian  tubules  in  the  course  of  their  development  have,  as  it 
were,  penetrated,  although  without  destroying  the  continuity  of  the  vascular 
endothelium.  It  is  by  the  intercrescence  of  the  tubules  and  of  the  endothelium 
that  the  sinusoidal  condition  is  established.  A  portion  of  the  original  channel 
remains  on  the  dorsal  side  of  the  Wolffian  body,  more  or  less  free,  V.  card.  We 
thus  learn  that,  owing  to  the  development  of  the  Wolffian  body,  the  posterior 
Cardinal  veins  as  such  disappear.  The  Wolffian  duct  is  always  on  the  ventral  side 
of  the  organ,  and  can  easily  be  traced  through  as  a  continuous  tube  from  section 
to  section.  In  the  figure  it  may  be  easily  found  in  the  left  mesonephros,  it  being 
there  the  lowermost  of  the  cavities  drawn  in  the  organ.  On  the  median  lower  sur- 
face of  the  Wolffian  body,  underneath  the  glomeruli,  is  an  accumulation  of  tissue, 
Gen,  the  anlage  of  the  genital  gland,  which  is  yet  very  slightly  advanced.  Below 
the  aorta  on  the  right  side  of  the  embryo  is  a  large  trunk  of  the  vena  cava  inferior, 
V.  C.  I,  on  its  way  past  the  right  dorsal  lobe  of  the  liver.  Near  the  aorta  on  the 
left  is  the  mesogastrium,  Om.  maj,  or  future  great  omentum,  by  which  the  stom- 
ach is  suspended  from  the  median  dorsal  wall  or  the  abdomen.  The  stomach,  Si, 
is  entirely  upon  the  left  side  of  the  body  and  is  directly  connected,  by  means  of 
the  anlage  of  the  lesser  omentum,  Om.  min,  with  the  liver.  The  walls  of  the 
stomach  are  constituted  by  the  splanchnopleure,  and,  therefore,  comprise  a 
layer  of  thickened  entoderm,  which  bounds  the  cavity  of  the  organ,  and  a  rela- 
tively thick  layer  of  mesoderm  which  forms  the  greater  part  of  the  wall,  and  the 
very  thin  superficial  mesothelium.  The  entoderm  is  a  smooth  layer  of  moderate 
thickness  composed  of  elongated  epithelial  cells.  It  forms  no  folds  and  shows  no 
trace  of  differentiation  into  gastric  glands.  In  the  mesenchyma  there  are  some 
capillary  blood-vessels.  The  mesothelium  is  thicker  than  over  the  liver  and 
somatopleure,  and  contains  crowded,  more  or  less  nearly  spherical  nuclei.  The 
liver  is  by  far  the  largest  organ  of  the  body.  It  takes  up  nearly  half  the  section. 
It  is  divided  into  four  main  lobes,  the  two  dorsal  and  two  ventral ;  two  on  the 
right  and  two  on  the  left.  The  reference  line,  Li,  runs  to  the  left  dorsal  lobe'. 
The  liver  consists  of  a  complicated  network  of  relatively  large  blood  sinusoids, 
the  spaces  between  which  are  entirely  occupied  by  the  embryonic  liver  cells.  Near 
the  median  plane,  between  the  ventral  lobes,  appears  the  gall-bladder,  G.  bl, 
which  is  cut  three  times.  The  liver  is  attached  in  the  median  ventral  line  to  the 
body- wall  and  to  the  base  of  the  umbilical  cord.  The  two  umbilical  veins  (com- 
pare Fig.  132)  enter  the  liver  directly  from  the  cord.  The  veins  are  originally 


SAGITTAL  SECTIONS  OF  EMBRYO   OF  12  MM.  205 

of  equal  size,  but  in  this  embryo  the  right  vein,  V.  U.  D,  has  already  become 
smaller  than  its  fellow,  V.  U.  S,  of  the  left  side,  and  in  later  stages  the  right  vessel 
is  found  to  have  disappeared  altogether.  The  two  veins  are  here  connected 
respectively  with  the  right  and  left  ventral  lobes  of  the  liver.  It  will  be  noticed 
that  the  right  and  left  sides  of  the  abdominal  cavity  are  completely  separated 
from  one  another,  and  that  there  is  a  special  part  of  this  cavity  shown  in  the 
section  between  the  stomach  and  the  right  dorsal  lobe  of  the  liver,  and  which  is 
known  as  the  lesser  peritoneal  space  or  cavity  of  the  omentum.  In  another  sec- 
tion the  lesser  peritoneal  space  is  found  to  connect,  by  means  of  a  very  small  and 
narrow  foramen  of  Winslow,  with  the  general  cavity  of  the  abdomen. 

The  Study  of  Sagittal  Sections,  Embryo  of  12  mm. 

From  a  sagittal  series  of  this  stage  many  significant  pictures  are  obtainable. 
Two  sections  only  have  been  selected  for  illustration  and  description :  first,  the 
median  section  of  the  head;  the  second  one  passing  through  the  principal  ceph- 
alic ganglia.  To  the  student  of  anatomy  these  sections  are  highly  instructive, 
for  they  exhibit  each,  or  in  a  single  picture,  many  important  fundamental  rela- 
tions of  the  brain,  cephalic  nerves,  and  other  structures  of  the  head. 

Median  Sections  of  the  Head. — The  plane  of  the  section  (Fig.  123)  is  almost 
exactly  median  for  the  region  of  the  hypophysis  and  infundibular  gland,  but  in 
the  region  of  the  spinal  cord  it  is  a  little  to  one  side;  hence  the  actual  plane  of  the 
section  is  slightly  oblique  to  the  true  median  plane  of  the  embryo.  The  present 
section  is  especially  instructive  as  regards  the  shape  of  the  brain  and  the  relations 
of  its  various  parts  to  one  another.  The  hind-brain  begins  at  the  spinal  cord, 
Sp.  c,  and  has  a  very  large  cavity,  the  fourth  ventricle,  Ven.  IV.  It  is  sepa- 
rated from  the  region  of  the  mid-brain  by  a  constriction  which  is  very  marked  on 
the  dorsal  side,  Isih.  The  constriction  is  known  as  the  isthmus.  It  is  always 
from  the  dorsal  side  of  the  isthmus  that  the  fourth  nerve  takes  its  origin.  It  is 
one  of  the  fixed  landmarks  of  the  brain.  The  mid-brain,  M.  B,  also  has  a  large 
cavity,  and,  as  a  whole,  forms  the  great  arch  which  corresponds  to  the  head-bend 
of  the  embryo.  It  passes  forward  and  downward,  without  any  very  definite  line 
of  demarcation  at  this  stage,  into  the  fore-brain,  the  cavity  of  which  is  larger  in 
diameter  than  that  of  the  mid-brain.  The  fore-brain  is  partially  subdivided  into 
two  regions;  the  anterior,  Pros,  is  the  prosencephalon  and  gives  rise  to  the  lat- 
eral outgrowths,  which  form  the  cerebral  hemispheres.  Already  the  deep  depres- 
sion separates  this  part  of  the  fore-brain  on  its  dorsal  side  from  the  posterior 
part,  which  is  termed  the  diencephalon.  The  limits  of  the  diencephalon  at  this 
stage  are  very  indistinct;  later  its  boundary  against  the  mid-brain  becomes 
clearly  marked  by  the  differentiation  of  the  epiphysis  and  posterior  commissure. 


206 


STUDY  OF  PIG  EMBRYOS. 


Floor 
/«/, 
Or, 
Som, 
,  In- 


SAGITTAL  SECTIONS  OF  EMBRYO  OF  12  MM.  207 

The  spinal  cord,  Sp.c,  forms  almost  a  right  angle  with  the  axis  of  the  hind-brain. 
This  angle  marks  and  corresponds  to  the  neck-bend  of  the  embryo.  On  its  dor- 
sal side  the  hind-brain  has  a  thin  ependymal  roof,  epen,  which,  however,  toward 
the  isthmus  thickens  considerably  to  produce  the  anlage,  Cbl,oi  the  median  por- 
tion of  the  cerebellum.  On  the  ventral  side  the  wall  of  the  fore-brain  varies  in 
appearance.  Where  the  section  is  exactly  median,  it  displays  the  raphe  or  floor- 
plate  of  the  region.  Where  it  is  off  the  median  plane,  it  shows  instead  the 
thicker,  lateral  wall  of  the  medulla  oblongata.  The  walls  of  the  mid-brain  on 
the  dorsal  side,  Q,  are  almost  uniform  in  thickness  and  texture.  They  are,  how- 
ever, later  to  be  differentiated  into  the  corpora  quadrigemina.  The  ventral  side 
of  the  mid-brain,  Fed,  is  considerably  thicker  than  the  dorsal,  and  forms  a 
strongly  marked  arch.  It  is  represented  in  the  adult  essentially  by  a  part  of  the 
peduncle  of  the  cerebrum.  The  floor,  Dien.  fl,  of  the  diencephalon  is  a  thin  mem- 
brane of  which  the  part  nearest  to  the  mid-brain  will  produce  the  mammary 
bodies,  and  the  part  further  from  the  mid-brain  the  tuber  cinereum.  It  has 
already  formed  a  special  outgrowth,  Inf,  the  anlage  of  the  infundibular  gland, 
which  extends  out  from  the  brain  and  arches  over  the  end  of  the  hypophysis,  Hyp. 
The  hypophysis  is  an  outgrowth  from  the  ectodermal  lining  of  the  mouth,  Or. 
Its  method  of  development  can  be  clearly  made  out  at  this  stage.  The  infundib- 
ular gland  in  older  embryos  extends  further  on  the  posterior  side  of  the  hypo- 
physis. Meanwhile  the  hypophysis  loses  all  connection  with  the  epithelium  of 
the  oral  cavity,  somewhat  as  does  the  otocyst  with  the  overlying  epidermis 
which  produces  it.  The  hypophysis  proper  and  the  infundibular  gland  undergo 
their  further  development  in  intimate  association.  The  result  of  their  differen- 
tiation is  the  pituitary  body,  which  is  really  a  duplex  organ.  Below  the  infundib- 
ular gland  the  wall  of  the  brain  shows  a  thickening,  Chi.  op,  which  can  be  fol- 
lowed through  in  the  series  laterally  until  it  connects  with  the  optic  stalk.  This 
thickening  of  the  brain-wall  in  later  stages  furnishes  the  passage  for  the  fibers  of 
the  optic  nerve,  and  is,  therefore,  the  anlage  of  the  optic  chiasma.  BetAveen  the 
infundibular  gland  and  the  optic  chiasma  extends  the  post-optic  laminae,  L.p.o. 
On  the  opposite  side  of  the  chiasma  follows  the  lamina  terminalis,  which  leads  us 
forward  to  the  wall  of  the  hemispheres,  H.  Underneath  the  hind-brain  extends 
the  large  basilar  artery,  A.  has;  at  its  posterior  end,  A.  has.  p,  the  basilar  artery 
is  joined  by  the  two  vertebral  arteries  from  the  fusion  of  which  it  is  really  pro- 
duced. Underneath  the  '  > rain  we  bc.-f*  the  o'pening  of  the  mouth,  Or,  from 
the  dorsal  side  of  which  springs  the  elongated  evagination  of  the  hypophysis. 
The  oral  cavity  runs  into  the  pharynx,  the  iloor  of  which  is  formed  in  part  by  the 
anlage  of  the  tongue,  Ton,  and  of  the  epiglottis,  Epgl,  a  rounded  eminence  very 
different  in  shape  at  this  stage  from  the  adult  epiglottis.  The  pharynx  can  be 


208  STUDY  OF  PIG  EMBRYOS. 

followed  along  until  it  passes  over  into  the  oesophagus,  Oe,  which,  however,  is  not 
well  shown,  as  the  section  passes  through  it  away  from  the  true  median  plane. 
Between  the  oesophagus  and  the  anlage  of  the  epiglottis  is  a  mound  of  tissue,  La, 
which  represents  the  lateral  wall  of  the  developing  larynx.  The  mound  is  sepa- 
rated from  the  anlage  of  the  epiglottis  by  a  deep  notch.  In  the  median  plane  the 
mound  is  filled  with  entoderm  which  forms  a  wide  plate  through  which  there  is 
only  a  narrow  opening  leading  down  into  the  trachea.  Finally,  we  see  from  the 
base  of  the  mandible  the  somatopleure,  Som,  extending  off  to  form  the  boundary 
of  the  pericardial  chamber.  The  figure  also  includes  a  presentation  of  the  infe- 
rior maxillary  vein,  V.  mx.  i,  and  of  the  thyroid  gland,  Thyr,  which  immediately 
overlies  the  main  trunk  of  the  ventral  aorta.  This  aorta  gives  off  on  either  side 
of  the  pharynx  three  principal  branches,  of  whch  the  smallest  is  the  base  of  the 
carotid  and  corresponds  to  the  third  aortic  arch.  The  second  and  third  branches 
are  much  larger  and  correspond  to  the  third  and  fourth  aortic  arches.  The  pul- 
monary aorta,  P.  Ao,  is  already  separated  from  the  main  aorta  of  the  body. 

Sagittal  Sections  of  the  Head  through  the  Principal  Ganglia. — The  section  (Fig. 
124)  is  to  one  side  of  the  median  plane.  It  exhibits  the  optic  nerve,  the  trigem- 
inal,acustico-facial,  petrosal,  jugular, and  nodosal  ganglia;  but,  on  the  other  hand, 
exhibits  little  of  the  brain,  there  being  only  a  shaving  from  the  lateral  wall  of  the 
fore-brain,  H,  and  a  section  of  the  widest  part  of  the  hind-brain  which  shows  the 
cavity  or  lateral  recess,  R.  L,  of  the  fourth  ventricle.  The  auditory  vesicle  is 
cut,  Ot.  It  is  formed  by  a  layer  of  epithelium  derived  from  the  ectoderm, 
although  now  not  connected  with  the  overlying  part  of  the  epidermis  by  the  in- 
vagination  of  which  the  otocyst  is  developed.  It  shows  a  narrow,  upward  pro- 
longation, the  anlage  of  the  ductus  endolymphaticus  (compare  Fig.  127).  The 
epithelial  otocyst  lies'  in  a  line  with  the  great  cephalic  ganglia  and  occupies 
its  invariable  and  permanent  position  behind  the  acustico-facial  ganglion, 
Ac.  F,  and  in  front  of  the  glosso-pharyngeal,  G.  petr.  The  position  of  the  otocyst 
makes  it  an  invaluable  landmark  in  the  study  of  sections  of  the  head.  Only  the 
lateral  portion  of  the  pharynx,  Ph,  appears.  It  forms  a  wide,  almost  slit-like 
diverticulum,  from  which  extend  further  laterally  the  first  and  second  entoder- 
mal  gill  pouches.  In  the  figure  can  be  seen  a  small  depression  extending  down- 
ward from  the  oesophageal  or  posterior  end  of  the  pharynx.  This  depression 
marks  the  beginning  of  the  second  cleft.  Nothing  is  seen  of  the  third  and  fourth 
clefts  in  this  section,  as  they  both  lip  nearer  the  median  plane.  The  pocket  or 
diverticulum  of  the  cervical  sinus,  Cer-v.  S,  lies  near  the  ganglion  nodosum,  G. 
nod.  From  its  appearance  it  might  easily  be  mistaken  for  the  section  of  a  gill 
cleft,  but  it  is  in  reality  lined  not  by  entoderm,  but  by  <x-t<>  lerrn,  and  its  cavity 
can  be  easily  traced  through  the  series  of  sections  of  the  exterior  of  the  embryo 


SAGITTAL  SECTIONS  OF  EMBRYO  OF  12  MM. 


209 


-  where  the  epithelium  lining  the  sinus  becomes  continuous  with  the  epidermis. 
Cephalad  from  the  sinus,  but  close  to  it,  lies  a  small  dark  rounded  mass,  the 
anlage  of  the  thymus  gland  (compare  Fig.  118,  Thm).  The  thymus  anlage 


Jug."'       G.jug.  Ph.  Ve.       Ac.F. 


EC. 


N.5. 


G.petr. 


G  .nod. 


Cerv.S. 


N.I  2. 


J"S- 


Md. 


N.op. 


FIG.  124. — PIG,  12.0  MM.     No.  7.     SAGITTAL  SECTION  25. 

Ac.F,  Acustico-facial  ganglion  complex.  Aur,  Auricle  of  the  heart.  Cerv.S,  Diverticulum  of  the  cervical 
sinus,  just  in  front  of  which  shows  the  anlage  of  the  thymus,  which  is  deeply  stained.  Cerv.6,  Sixth  cervi- 
cal nerve.  Cce,  Ccelom  around  the  heart  or  pericardia!  cavity.  G.jug,  Ganglion  jugulare  of  the  vagus 
nerve.  G.nod,  Ganglion  nodosum  of  the  vagus  nerve.  G.petr,  Ganglion  petrosum  of  the  glossopharyngeal 
nerve.  G.tri,  Ganglion  of  the  trigeminus  nerve.  H,  Lateral  wall  of  the  cerebral  hemisphere.  Jug, 
Jugular  vein.  (Jug* ',  Behind  the  trigeminus.  Jug" ,  Branch  in  front  of  the  trigeminus.  fag'",  Main 
stem  behind  the  vagus.  Jug"" ,  Main/item  descending  to  join  the  duct  of  Cuvier. )  m,  An  undetermined 
structure,  probably  the  anlage  of  a  lingual  muscle.  Md,  Mandible.  N.$,  Root  of  the  fifth  or  trigeminal 
nerve.  N.op,  Optic  nerve.  N.I2,  Twelfth  or  hypoglossal  nerve.  Ot,  Otocyst.  Ph,  Pharynx.  R.L, 
Recessus  lateralis  of  the  fourth  ventricle.  „  Ve,  Small  branch  of  the  jugular  vein.  Vent,  Ventricle  of  the 
heart.  X  22  diams. 


is  produced  by  proliferation  of  the  entodermal  cells  on  the  anterior  side  of 
the  third  cleft,  and  is  penetrated  by  blood-vessels  which  seem  to  be  sinusoids, 
although  their  history  has  not  been  worked  out.  The  great  vein  of  the  head, 

14 


210  STUDY  OF  PIG  EMBRYOS. 

which  for  convenience  we  may  term  the  jugular, — although  the  application  of  this 
name  to  the  vein  in  its  present  condition  is  somewhat  inexact, — is  cut  several 
times,  owing  to  its  irregular  course.  Its  main  stem,  Jug"'1 ',  arises  nearly  verti- 
cally through  the  cervical  region  and  is,  relatively  to  the  size  of  the  embryo,  of 
huge  diameter.  It  continues  upward,  Jug'" ,  along  the  dorsal  side  of  the  vagus 
to  about  half-way  between  the  ganglion  nodosum  and  ganglion  jugulare.  At  that 
point  the  vessel  curves  inward  and  forward,  and,  therefore,  is  not  encountered 
again  in  this  section  until,  having  bent  upward  again,  it  shows,  Jug',  on  its  way 
past  the  trigeminal  ganglion.  A  branch  of  the  jugular  is  cut  just  above  the  gan- 
glion, Jug",  and  another  small  and  probably  not  very  important  branch  is  shown 
at  Ve. 

The  nerves  are  shown  as  follows :  The  optic  nerve,  N.  op,  still  has  its  central 
cavity,  which,  nearer  the  median  plane,  opens  into  the  third  ventricle  of  the  brain, 
and  in  the  section  resembles  in  shape  an  inverted  L).  On  the  side  of  the  nerve 
toward  the  mouth  there  is  a  deep  notch, — the  section  of  the  choroid  fissure.  The 
trigeminal  ganglion,  G.  tri,  is  very  large,  and  its  trilobate  form  is  clearly  indi- 
cated by  the  figure.  The  lobe  to  which  the  reference  line,  G.  tri,  runs  gives  off 
the  ramus  ophthalmicus ;  the  lobe  nearest  the  jugular  gives  off  the  ramus  maxil- 
laris  inferior,  while  the'middle  lobe  gives  off  the  ramus  maxillaris  superior.  From 
the  ganglion  the  fibers  and  nerve-cells  extend  upward  to  form  the  root,  N.  5, 
which  joins  the  hind-brain  at  a  characteristic  point, — namely,  at  the  summit  of 
the  Varolian  bend  and  where  the  hind-brain  is  widest  (compare  Figs.  113  and  125). 
By  its  great  size  and  by  its  topographical  association  with  the  lateral  apex  of 
the  recessus  lateralis  of  the  fourth  ventricle,  the  trigeminal  ganglion  may  always 
be  readily  identified  in  sections  of  embryos.  The  acustico-facial  ganglia,  Ac.  F, 
may  also  be  readily  determined  by  their  typical  position  immediately  in  front  of 
the  otocyst,  Ot.  But  it  is  quite  difficult  to  identify  the  four  components  of  this 
complex  structure, — namely,  i°,  the  motor  root  of  the  facial  nerve;  2°,  the  facial 
or  geniculate  ganglion ;  3°,  the  vestibular  ganglion ;  4°,  the  cochlear  ganglion.  In 
figure  124  three  divisions  are  shown.  The  large,  darkly  stained  division,  to 
which  the  reference  line,  Ac.  F,  runs,  and  which  lies  nearest  to  the  otocyst,  is 
the  vestibular  portion  of  the  acoustic  ganglion;  the  small,  light  area  occupying 
a  middle  position  in  the  inferior  part  of  the  complex  is  the  motor  division  of  the 
seventh  nerve,  or  lateral  root  of  the  facial ;  it  can  be  followed  to  the  brain,  which 
it  enters  as  four  bundles  of  fibers ;  its  path  of  entrance  is  shown  better  in  frontal 
sections  (Fig.  1 26,  t.m}.  Just  in  front  of  the  facial  motor  root  lies  a  second  smaller 
dark  mass,  the  geniculate  ganglion  of  the  facial,  with  an  upward  prolongation, 
the  sensory  root.  The  ninth  or  glossopharyngeal  nerve  is  represented  by  the  gan- 
glion petrosum,  G.  petr,  and  its  ascending  sensory  root.  This  nerve  may  be 


FRONTAL  SECTIONS  OF  EMBRYO   OF  12  MM.  211 

quickly  identified  because  it  is  the  first  behind  the  otocyst.  The  upper  ganglion 
of  this  nerve  the  so-called  Ehrenritter's  ganglion,  is  represented  by  an  accumula- 
tion of  cells  ii  the  upper  part  of  this  root.  As  regards  the  tenth  nerve,  or  vagus, 
both  its  ganglia  and  the  fibrous  trunk  connecting  them  are  shown.  The  upper 
or  jugular  ganglion,  G.  jug,  is  nearly  on  a  level  with  the  otocyst,  while  the  lower 
or  nodosal,  G.  nod,  lies  near  the  cervical  sinus.  To  the  nerve-trunk  between 
the  two  ganglia  are  adjoined  the  fibers  of  the  eleventh  or  spinal  accessory  nerve, 
which  does  not  otherwise  appear  in  this  section.  A  small  piece  only  of  the  hypo- 
glossal  nerve  can  be  seen,  N.  12.  The  space  occupied  by  this  nerve  is  blank  in 
the  engraving;  in  the  specimen  it  shows  horizontal  fibers. 

The  Study  of  Frontal  Sections,  Embryo  of  12  mm. 

The  frontal  series  has  special  value  for  the  study  of  the  hind-brain  and  asso- 
ciated structures,  as  the  plane  of  the  section  is  approximately  at  right  angles  to 
the  axis  of  the  hind-brain. .  It  also  furnishes  instructive  pictures  of  the  relations 
of  developing  vertebrae  and  nerves. 

Portions  of  three  sections  illustrating  the  structure  of  the  hind-brain  and 
associated  parts  are  given  below.  The  following  remarks  on  the  hind-brain  are 
intended  to  make  the  significance  of  these  sections  clearer.  The  wall  of  the  hind- 
brain  is,  of  course,  produced  by  the  development  of  the  wall  of  the  medullary 
tube.  Its  most 'striking  peculiarity  is  the  enormous  expansion  of  the  deck- 
plate,  which  forms  the  very  wide  epithelial  layer,  epen,  the  so-called  ependymal 
roof  of  the  fourth  ventricle.  Ijt  starts  from  the  upper  edge  of  the  dorsal  zone, 
Fig.  125,  D.  Z,  and  forms  a  wide  arch  which  is  covered  in  externally  by  a  rather 
thin  layer  of  mesoderm,  mes,  and  the  nearby  epidermis,  EC,  of  the  embryo.  The 
covering  is  so  slight  in  development  at  this  stage  that  in  the  fresh  specimen  the 
roof  of  the  fourth  ventricle,  including  its  coverings,  appears  as  a  translucent  mem- 
brane through  which  we  can  readily  distinguish  the  great  cavity  of  the  fourth  ven- 
tricle itself.  The  expanse  of  the  ependymal  arch  is  greatest  at  the  region  of  the 
trigeminal  root.  From  there  backward  toward  the  spinal  cord  its  expanse  gradu- 
ally diminishes.  In  correspondence  with  the  growth  of  the  deck-plate  the  lateral 
walls  of  the  medullary  tube  become  bent  outward  and  downward,  so  that,  though 
they  remain  near  together  on  their  ventral  side,  where  they  are  united  by  the  floor- 
plate  or  median  raphe  (Fig.  127,  raph),  yet  their  upper  dorsal  edges  are  far  apart. 
In  consequence  of  this  change  of  their  position  the  original  lateral  walls  appear  as 
the  floor  of  the  hind-brain,  and  we  recognize  in  them  the  anlages  of  the  medulla 
oblongata.  We  distinguish  here,  as  everywhere  in  the  medullary  wall,  the  dorsal 
and  ventral  zones.  The  ventral  zone  is  intimately  united  with  its  fellow  by  the 
short  median  raphe.  Between  them  is  a  deep  fissure  (Fig.  1 26,  /),  which  is  never 


212 


STUDY  OF  PIG  EMBRYOS. 


epen 


D.Z. 
T.S. 


wholly  obliterated.  The  floor-plate  undergoes  a  great  development  in  later 
stages  and  is  transformed  into  the  median  raphe  of  the  adult  nVdulla.  The 
lateral  or  morphologically  dorsal  limit  of  the  ventral  zone  is  miked  by  the 
exit  of  the  lateral  roots  (Fig.  125,  L.  R).  The  ventral  limit  of  the  (fcrsal  zone  is 
marked  by  the  entrance  of  the  sensory  or  ganglionic  fibers  (Fig.  125,  G.  tri;  Fig. 
1 26,  Fac).  Toward  the  dorsal  side  the  dorsal  zone  gradually  thins  out  and  passes 

over  into  the  ependymal,  epen.  The  great 
development  of  the  lateral  roots  is  perhaps 
the  most  important  single  characteristic  of  the 
medulla  oblongata.  They  furnish  the  principal 
motor  or  efferent  nerve-tracts  of  the  brain  and 
form  an  important  constituent  part  of  four 
nerves:  first,  the  trigeminal  or  fifth;  second, 
the  facial  or  seventh ;  third,  the  glosso-pharyn- 
geal  or  ninth;  and  fourth,  the  vagus  or  tenth. 
There  are  no  lateral  roots  known  to  occur  an- 
terior to  the  medulla  oblongata,  unless  possibly 
the  fourth  nerve,  the  relations  of  which  in 
many  respects  are  peculiar,  should  turn  out  to 
be  a  lateral  root.  In  the  spinal  cord  we  find 
lateral  roots  in  the  upper  cervical  region,  and 
it  is  not  improbable  that  they  may  yet  be 
found  associated  with  the  dorsal  roots  of  spinal 
nerves  lower  down.  But  even  in  the  cervical 
cord  the  lateral  roots  attain  but  a  slight  devel- 
opment. The  contrast  with  other  portions  of 
the  central  nervous  system  makes  the  great 
development  of  the  lateral  roots  in  the  medulla 
oblongata  all  the  more  striking.  The  dorsal 
zone  of  the  hind-brain  lags  considerably  behind 
the  ventral  zone  in  its  development,  and  at  all 
stages  the  ventral  zone  forms  a  larger  propor- 
tion of  the  medulla  than  does  the  dorsal  zone. 

Section  through  the  Trigeminal  Roots. — The  section  passes  through  the  widest 
part  of  the  hind-brain,  the  cavity  of  which  is  enormously  distended.  It  is 
bounded  on  the  dorsal  side  only  by  the  very  thin  ependymal  roof,  epen,  which 
does  not  form  any  part  of  the  true  nervous  structure,  although  it  passes  into  and 
is  directly  continuous  with  the  dorsal  zone,  D.  Z,  which  is  thus  seen  to  be  only  a 
thickened  portion  of  the  wall  of  the  neural  tube,  just  as  the  ependyma  is  the 


L.R. 

G.tri. 


Card. 


FIG.  125. — PIG,  12.0  MM.  FRONTAL 
SERIES  6,  SECTION  284. 

Card,  Anterior  cardinal  vein.  D.Z,  Up- 
per portion  of  the  dorsal  zone  of  His. 
EC,  Ectoderm.  epen,  Ependymal 
roof  of  the  fourth  ventricle.  G.tri, 
Ganglion  trigemini.  L.R,  Lateral 
root  of  the  trigeminal  nerve,  mes, 
Mesenchyma.  T.S,  Tractus  soli- 
tarius  of  W.  His.  X  22  dkms. 


FRONTAL  SECTIONS  OF  EMBRYO  OF  12  MM. 


213 


attenuated  deck-plate.  The  trigeminal  ganglion,  G.  tri,  is  very  large  and  sends 
its  sensory  fibers  upward  into  the  dorsal  zone  to  form  there  a  distinct  bundle  of 
nerve-fibers  which  persists  throughout  life  and  is  known  in  the  adult  as  the 
tractus  aolitarius,  T.  S.  The  other  root  of  the  nerve,  L.  R,  is  lateral.  It  lies 
below  the  ganglion  near  the  median  plane.  Its  fibers  arise  from  neuroblasts  in 
the  ventral  zone  and  gather  together  as  a  distinct  bundle  which  starts  near  the 
median  line,  takes  a  curving  course 
through  the  ventral  zone,  and  makes 
its  exit  from  the  medullary  wall  at  the 
dorsal  limit  of  the  zone.  It  has  a  strik- 
ing resemblance  to  the  root  of  the  facial 
nerve.  We  do  not  yet  know  whether 
such  a  course  of  the  fibers  is  character- 
istic of  all  lateral  roots  or  only  of  the 
trigeminal  and  facial  roots.  On  the 
medial  side  of  the  trigeminal  ganglion 
is  a  large  vein,  Card,  the  anterior  car- 
dinal vein,  which  by  island  formation  is 
to  migrate  to  the  outside  of  the  gan- 
glion to  form  a  portion  of  the  permanent 
jugular  trunk.  In  the  median  line  in 
the  mesenchyma  immediately  below 
the  raphe  is  the  section  of  the  basilar 
artery,  and  considerably  below  that  is 
the  small  section  of  the  notochord 
which  it  is  very  difficult  to  distinguish 
with  a  low  power.  Between  the  noto- 
chord and  the  cardinal  vein  is  the  sec- 
tion of  the  carotid  artery. 

Section  through  the  Acustico- facial 
Ganglion. — In  this  section  (Fig.  126) 
the  thickened  ventral  wall  of  the  hind- 
brain  (i.  e.,  the  anlage  of  the  medulla 
oblongata)  is  not  spread  out  nearly 
horizontally,  as  in  the  trigeminal  region,  but  rises  obliquely  on  either  side 
from  the  median  line.  The  right  and  left  sides  of  the  medulla  are  divided 
from  one  another  by  a  deep  median  fissure,  /.  In  the  median  line  we  see  also 
the  basilar  artery,  A.  has,  and  still  lower  the  wide,  slit-like  pharynx,  Ph,  the 
outer  portion  of  which  ascends  obliquely  toward  the  jugular  vein,  Jug.  The 


G.gen. 


N.I  2. 


Mx.i. 


FIG.  126. — PIG,  12.0  MM.  FRONTAL  SERIES  6, 
SECTION  340. 

A. has,  Arteria  basilaris.  D.Z,  Dorsal  zone  of  the 
medulla  oblongata.  EC,  Ectoderm,  epen,  Epen- 
dymal  roof  of  the  fourth  ventricle,  f,  Median 
fissure  of  the  medulla  oblongata.  Fac,  Sensory 
root  of  the  facial  nerve.  G.gen,  Geniculate 
ganglion  of  the  facial  nerve.  G.vtsf,  Ganglion 
vestibuli  of  the  acoustic  nerve,  fug,  Jugular 
vein,  mes,  Mesenchyma.  Mx.i,  Inferior  max- 
illary branch  of  the  trigeminal  nerve.  N.I2, 
Hypoglossal  nerve.  Ph,  Pharynx,  t.m,  Motor 
tract  of  facial  nerve.  X  22  diams. 


214 


STUDY  OF  PIG  EMBRYOS. 


epen. 


Md.obl. 


ascending  lateral  part  of  the  pharynx  is  a  portion  of  the  first  gill  pouch  or  future 
Bustachian  tube,  and  is  quite  clearly  marked  off  from  the  pharynx  proper*  by 
its  oblique  direction.  Of  the  acustico-facial  ganglion  complex  the  section  shows 
four  parts:  the  ganglion  vestibuli,  G.  vest;  the  geniculate  ganglion,  G.  gen;  the 
sensory  root,  Fac,  of  the  facial  nerve  arising  from  the  geniculate  ganglion  and 
entering  the  brain  to  form  there  a  distinct  fiber-tract  which  is  oval  in  the  section 

and  lies  just  below  the  entering  vesti- 
bular  fibers,  and  is  clearly  indicated  in 
the  drawing;  and,  finally,  the  motor 
tract,  t.  m,  of  the  facial  nerve.  This 
tract  is  a  very  distinctly  marked  bun- 
dle of  nerve-fibers  which  arise  from 
neuroblasts  of  the  ventral  zone,  tra- 
verse that  zone  almost  horizontally, 
then  bend  downward  and  pass  out  from 
the  brain-wall,  appearing  as  the  lateral 
root  of  the  facial  nerve.  The  root  runs 
first  toward  and  then  past  the  genicu- 
late ganglion.  The  jugular  vein,  Jug, 
lies  outside  of  the  ganglia;  not,  as  does 
the  earlier  cardinal  vein,  inside.  In  the 
mandible  below  the  pharynx  appear 
two  nerves.  Of  these,  the  upper  is  the 
hypoglossal,  N '.  12,  which  lies  near  the 
angle  formed  by  the  jugular  of  the  first 
gill  cleft  with  the  pharynx.  The  lower 
of  the  two  nerves,  MX.  i,  is  the  inferior 
maxillary. 

Section  through  the  Otocyst. — The 
figure  is  from  a  section  not  far  from 
the  last.  The  hind-brain  has  narrowed 
considerably;  its  thickened  floor,  Md. 
obi,  the  anlage  of  the  medulla  oblon- 

gata,  rises  steeply  from  the  median  line.  Its  ependymal  roof,  epen,  is  less 
expanded  than  in  figures  126  and  127.  It  forms  a  sharp  angle  in  the  dorsal 
median  line.  The  median  ventral  fissure  between  the  two  sides  of  the 
medulla  is  deeper  than  further  forward.  The  pharynx,  Ph,  is  wide  and  has 
expanded  laterally  into  the  common  beginning  of  the  first  and  second  gill 
pouches.  Between  the  pharynx  and  the  raphe  the  basilar  artery,  A.  has, 


PA. 


FIG.  127. — PIG,  12.0  MM.    FRONTAL  SERIES  6, 

SECTION  380. 

A.bas,  Basilar  artery.  Coch,  Cochlea.  D.e,  Ductus 
endolymphaticus.  epen,  Ependyma.  Fac. in, 
Motor  division  of  the  facial  nerve.  Jug,  Jugu- 
lar. Md.obl,  Medulla  oblongata.  Ph,  Pharynx. 
raph,  Median  raphe  of  the  medulla  oblongata. 
S.c,  Anlage  of  the  semicircular  canals.  Ve,  Vein. 
X  22  diams. 


FRONTAL  SECTIONS  OF  EMBRYO  OF  12  MM.  215 

has  been  cut  transversely.  Below  it  and  near  the  pharynx  is  the  small  noto- 
chord,  which,  however,  can  only  be  clearly  recognized  with  the  higher  power,  and 
is,  therefore,  not  represented  in  this  or  the  preceding  figure.  The  otocyst  is  a 
large  epithelial  vesicle  with  three  well-marked  divisions:  first,  the  common 
chamber,  5.  c,  out  of  which  the  three  semicircular  canals  are  to  be  differentiated. 
Second,  a  slender  canal,  D.e,  which  one  easily  identifies  as  the  anlage  of  the  ductus 
endolymphaticus.  It  lies  between  the  semicircular  canal  and  the  wall  of  the 
medulla  oblongata.  Third,  the  long,  curving,  but  not  spiral  cochlea.  The  com- 
mon chamber  formed  by  the  union  of  these  divisions  is  later  subdivided  to  form 
the  upper  utriculus  and  lower  sacculus.  Outside  the  cochlea  lies  the  cross- 
section  of  the  jugular  vein,  just  below  which  is  the  section  of  the  motor  portion, 
Fac.  m,  of  the  facial  nerve.  The  sensory  portion  of  the  facial  nerve  at  this  stage 
is  much  smaller,  and  runs  only  a  short  distance  downward  from  the  geniculate 
ganglion  and  is  entirely  separate  from  the  motor  portion.  The  morphological 
constitution  of  the  facial  nerve  is  still  very  obscure,  and  a  satisfactory  account 
of  its  development  is,  for  the  present,  impossible. 

Section  through  the  Dorsal  Vertebra  (Fig.  1 28). — Owing  to  the  curvatureof  the 
embryo  the  spinal  cord  is  cut  twice;  once,  Sp.  cf ,  toward  the  head  end  of  the  em- 
bryo, and  again,  Sp.  c",  lower  down  toward  the  tail  end.  Alongside  the  sections 
of  the  spinal  cord  appear  the  large,  darkly  stained  masses  of  the  ganglia,  G.  The 
section  also  passes  through  the  bases  of  the  anterior  limbs,  A.  L,  in  one  of  which 
we  can  see  one  of  the  branches,  N.  br,  of  the  brachial  plexus.  Between  the  two 
pieces  of  the  spinal  cord  of  the  section  the  plane  passes  on  the  ventral  side  of  the 
spinal  cord  and  shows  the  series  of  vertebral  formations,  together  with  the  nerve- 
roots,  N",  the  intersegmental  arteries,  A.  i.  s,  and  the  segmental  veins,  small 
vessels  which  lie  close  to  the  intersegmental  arteries.  The  nerves  are  sections  of 
the  dorsal  root  below  the  ganglia.  Each  nerve  has  a  distinct  outline  and  is  partly 
penetrated  by  ingrowing  mesenchymal  cells  which  subdivide  the  nerve  into 
rounded  fiber  bundles.  In  each  bundle  the  nerve-fibers  appear  as  fine  dots, 
which,  however,  by  the  use  of  the  fine  ad justmen^  can  be  followed  up  and  down 
through  the  section,  and  thus  identified  as  fibers.  The  single  fibers  are  more  or 
less  isolated  from  one  another,  and  between  them  are  delicate  threads,  the  nature 
of  which  is  not  known.  Between  the  adjacent  rounded  bundles  of  fibers  there  is 
often  a  distinct  space.  The  vertebral  anlages,  Vert,  are  formed  entirely  from 
condensed  mesenchyma,  and  therefore  stand  out  somewhat  conspicuously  in  the 
section,  owing  to  their  darker  staining.  Each  anlage  is  bow-shaped,  the  con- 
cavity of  the  bow  facing  toward  the  tail  of  the  embryo.  The  ends  of  the  bow  pass 
behind  the  nerve-trunk  of  the  segment  to  which  the  vertebral  anlage  belongs. 
The  anlages  extend  completely  across  the  median  line,  and  by  following  through 


Sp.  c." 


FIG.  128. — PIG,  12.0  MM.  FRONTAL  SERIES  6,  SECTION  572. 

A.i.s,  Intersegmental  artery.  A.L,  Anterior  limb,  cin,  Cinerea  of  spinal  cord.  EC,  Ectoderm.  G,  Ganglion. 
mes,  Mesoderm.  N1 '  ,Nf> ',  Nerves.  JV.br,  Nerve-branch  of  brachial  plexus.  Sp.c' ',  Cephalad  portion  of 
spinal  cord.  Sp.c" ,  Caudad  portion  of  spinal  cord.  V.arch,  Anlage  of  arch  of  vertebra.  Vert,  Anlage 
of  the  body  of  a  vertebra.  X  22  diams. 

216 


EMBRYO  OF  9  MM.  217 

in  the  series  of  sections,  it  may  be  found  that  the  condensed  mesenchyma  sur- 
rounds the  notochord,  which,  therefore,  passes  through  the  central  portion  of 
each  vertebral  anlage.  The  vertebrae  at  this  stage  are  entirely  without  any 
distinct  limitation  and  merge  into  the  surrounding  loose  mesenchyma.  Near 
the  anterior  border  of  each  nerve-trunk,  and  usually  somewhat  toward  the  me- 
dian side  of  it,  lie  the  intersegmental  vessels,  which  are  of  small  size  and  vary 
greatly  in  their  exact  position  and  number,  according  as  they  are  more  or  less 
branched.  Between  the  ends  of  the  vertebral  bows  outside  of  the  nerve-trunks 
can  be  seen  with  higher  power  clusters  of  elongated  cells  with  developing  muscle- 
fibers  which  are  here  still  segmentally  arranged  between  the  processes  of  the 
developing  vertebrae. 

Pig  Embryo  of  9  mm. 

Pig  embryos  of  this  stage  supplement  very  instructively  those  of  1 2  mm.  It 
will,  of  course,  be  advantageous  for  the  student  to  prepare  serial  sections  himself. 
When  that  is  not  possible,  there  should  at  least  be  sections  prepared  for  the  lab- 
oratory which  the  student  may  examine.  Five  sections  are  illustrated  and 
described  below.  They  have  been  chosen  to  supplement  the  descriptions  of  the 
sections  of  the  pig  of  12  mm.,  and  they  will  be  found  to  illustrate  certain  funda- 
mental morphological  relations  in  the  embryo  more  clearly  than  older  stages. 

Transverse  Section  through  the  Region  of  the  Branchial  Arches. — The  bran- 
chial arches  are  much  more  conspicuous  at  this  stage  than  in  later  ones,  being 
separated  from  one  another  by  deep  ectodermal  depressions,  figure  1 29,  /,  //,  ///, 
IV;  and,  although  ///  and  IV  are  already  being  turned  in,  preparatory  to  the 
formation  of  the  cervical  sinus,  they  are  still  distinct  and  their  order  in  the  series 
is  evident.  The  section  (Fig.  129)  shows  on  the  dorsal  side  the  spinal  cord,  in 
which  we  can  already  recognize  the  subdivision  into  dorsal  zone,  D.  Z,  and  ven- 
tral zone,  V.  Z.  To  the  dorsal  zone  is  appended  the  dorsal  root ;  from  the  middle 
of  the  ventral  zone  comes  off  the  ventral  root  of  a  cervical  nerve,  N.  Just 
between  the  dorsal  root  and  the  wall  of  the  spinal  cord  can  be  seen  the  section  of 
the  accessory  nerve.  The  secondary  segment,  My,  is  sharply  defined  and  has  a 
distinct  growing  edge  showing  at  its  upper  limit  in  the  section.  The  inner  leaf 
of  the  secondary  segment  is  stained  more  lightly  than  the  neighboring  tissue 
corresponding  to  the  modifications  which  the  cells  are  undergoing  preparatory 
to  their  change  into  young  muscle-fibers.  In  the  12  mm.  pig  in  this  region  the 
cells  of  the  muscle  plate  have  already  broken  apart  and  no  distinct  plate  can  any 
longer  be  recognized.  Below  the  muscle  plate  follows  the  section  of  the  jugular 
vein,  Jug.  Lower  down  and  in  the  median  line  we  have  the  section  of  the 
pharynx,  Ph,  lined  by  the  epithelial  entoderm.  The  pharynx  is  surrounded  by 


218 


STUDY  OF  PIG  EMBRYOS. 


the  very  large  aortic  vessels,  which  start  from  -the  ventral  side  of  the  pharynx 
and  pass  upward  along  its  sides  to  join  the  descending  aorta,  Ao.  d.  4,  at  about 
the  level  of  the  jugular  veins.  The  vessels  shown  are  the  fourth  aortic  arches. 


oif. 


F.B. 


D.Z. 


Nch. 


11IJV. 


FIG.  129. — PIG,  9.0  MM.     TRANSVERSE  SERIES  9,  SECTION  171. 

Ao.d.4,  Descending  aorta  receiving  the  right  fourth  aortic  arch,  cl.lll,  Third  entodermal  gill  cleft.  D.Z, 
Dorsal  zone  of  spinal  cord.  F.B,  Fore-brain.  Hy,  Hyoid  branchial  arch.  Jug,  Jugular  vein.  Aldb, 
Mandibular  branchial  arch.  My,  Muscle  plate.  JV,  Nerve.  Nch,  Notochord.  Olf,  Olfactory  plate.  Ph, 
Pharynx.  V.Z,  Ventral  zone  of  spinal  cord.  /,  II,  III,  I V,  First  to  fourth  ectodermal  gill  clefts.  X  35 
diams. 

Their  symmetry  and  their  relations  to  the  pharynx  are  beautifully  demon- 
strated in  this  section.     Below  the  aorta  we  find  a  section  of  the  third  internal 


EMBRYO  OF  p  MM.  219 

gill  cleft,  d.  Ill,  a  narrow,  slit-like  cavity  lined  by  entoderm.  On  the  left-hand 
side  of  the  embryo  the  junction  of  the  entoderm  of  the  internal, pouch  with  the 
ectoderm  is  shown.  The  two  germ-layers  have  united  to  form  a  typical  closing 
plate.  Above  the  third  gill  cleft  the  outline  of  the  embryo  shows  a  deep  depres- 
sion which  is  due  to  the  commencing  formation  of  the  cervical  sinus.  From  the 
upper  end  of  this  depression  runs  upward  the  ectodermal  fourth  cleft,  and  from 
its  lower  part  extends  downward  the  ectodermal  third  cleft.  Between  the  third 
and  fourth  clefts  the  external  surface  of  the  embryo  protrudes  somewhat.  This 
protuberance  corresponds  to  the  so-called  third  branchial  arch.  Between  the 
third  external  cleft  and  the  second,  II,  is  a  still  greater  protuberance  on  the  out- 
side of  the  embryo.  This  marks  the  third  branchial  arch.  The  third  aortic 
arches  are  somewhat  imperfectly  shown,  but  the  connection  of  the  left  third 
arch  with  the  central  aorta  appears.  Between  the  second  and  first  external 
clefts  we  have  the  second  or  hyoid  branchial  arch,  Hy;  and,  similarly,  between 
the  first  or  auditory  cleft,  /,  and  the  oral  fissure,  which  separates  the  head  from 
the  body  of  the  embryo,  we  have  the  very  large  and  protuberant  mandibular 
arch,  Mdb.  The  head  of  the  embryo  is  completely  separated  in  this  section 
from  the  body.  It  shows  the  cavity  of  the  fore-brain,  F.  B,  bounded  by  the  ecto- 
derm of  the  medullary  wall,  and  on  one  side  also  shows  the  thickening  of  the  epi- 
dermis, Olf,  which  forms  the  olfactory  plate  or  plakode,  which  is  to  become  the 
lining  of  the  nasal  pit. 

Sagittal  Section  to  the  Right  of  the  Median  Plane. — In  the  accompanying 
figure  130  the  cephalic  end  of  the  embryo  is  omitted;  a  portion  of  the  heart,  the 
entire  length  of  the  Wolifian  body,  and  the  tail  are  included.  The  dorsal  outline 
of  the  embryo  forms  a  characteristic  curve.  A  long  series  of  spinal  ganglia,  G, 
are  shown  arranged  in  regular  succession  and  following  the  curvature  of  the  back. 
The  ganglia  are  easily  recognizable  by  their  dark  staining ;  each  of  them  is  so  large 
as  to  occupy  at  least  four-fifths  of  the  length  of  the  segment  to  which  it  belongs. 
The  boundaries  between  the  adjacent  primitive  segments  are  indicated  by  the 
positions  of  the  intersegmental  arteries,  A.  is.  Even  when  their  cavities  do  not 
show,  the  position  of  these  vessels  is  marked  by  the  darker  line  of  tissue.  The 
origin  of  one  of  these  intersegmental  vessels  from  the  dorsal  aorta,  Ao,  is  indi- 
cated in  the  lower  part  of  the  figure.  The  Wolman  body,  W.  b' ,  W.  b",  extends 
from  the  level  of  the  lungs  and  liver  well  down  toward  the  pelvic  end  of  the  em- 
bryo. Its  ventral  limit  is  marked  by  the  body-cavity,  Coe,  and  it  is,  of  course, 
covered  by  a  layer  of  mesothelium,  msth,  which  here,  as  everywhere  and  at  all 
stages,  forms  the  boundary  of  the  coelom.  In  the  Wolffian  body  we  distinguish 
readily  numerous  sections  of  the  epithelial  Wolffian  tubules,  and  toward  the 
ventral  side  of  the  organ  the  characteristic  glomeruli,  Glo.  Between  the  glom- 


220 


G. 


STUDY  OF  PIG  EMBRYOS. 

Ao.D.       Ptd.     D.ven.          V.s.         Au. 


A. is. 


W.b/ 


Soni. 


V.  mm . 


msth. 


FIG.  130. — PIG,  9.0  MM.     SAGITTAL  SERIES  53,  SECTION  213. 

A,  Entodermal  cavity  (probably  gall-bladder).  A.is,  Intersegmental  artery.  All,  Allantois.  Ao,  Median  aorta. 
Ao.D,  Descending  aorta.  Art.v,  Arteria  vitellina.  Au,  Auricle.  Clo,  Cloaca.  Cce,  Coelom.  D.ven, 
Ductus  venosus.  Ent,  Entoderm.  G,  Ganglion.  Glo,  Glomerulus.  Li,  Liver,  msth,  Mesothelium. 
Pul,  Lung.  Seg,  Segment.  Sow,  Somatopleure.  Sp.c,  Spinal  cord.  Uin.iv' y  Upper  wall  of  umbilicus. 
Um.w",  Lower  wall  of  umbilicus.  Ve,  Vein  in  liver.  Ven,  Ventricle  of  heart.  Vil,  Vlllus.  V.msn, 
Vena  mesonephrica.  V.p,  Portal  vein.  V.s,  Valvula  sinistra.  W.b' ,  W.b" ,  Wolffian  body.  X  22 
diams. 


EMBRYO  OP  p  MM.  221 

eruli  and  the  .mesothelium  there  is  a  layer  of  mesenchyma,  but, between  the 
tubules  there  is  little  tissue,  the  intertubular  spaces  being  almost  entirely  occu- 
pied by  sinusoids  developed  from  the  cardinal  vein.  The  larger  sinusoid  or 
venous  space,  V.  msn,  is  due  to  the  section  of  the  venous  trunk  which  joins  the 
lower  end  of  the  vena  cava  inferior,  and  is  known  as  the  mesonephric  vein.  In 
the  upper  part  of'the  figure  we  encounter  a  section  of  the  descending  aorta,  Ao.  d, 
and  of  the  lungs,  Pul,  or  pulmonary  anlage.  It  consists  of  a  ring  of  entoderm 
bounding  the  central  cavity  and  enclosed  by  a  thicker  layer  or  mesenchyma, 
which,  again,  is  bounded  by  a  layer  of  mesothelium.  The  space  or  coelom  about 
the  lung  is  shown  in  the  figure  to  be  continuous  with  the  coelom  of  the  abdominal 
region.  On  the  ventral  side  we  have  the  heart  partly  shown,  the  ventricle,  Ven, 
being  so  cut  as  to  exhibit  the  trabecular  structure  of  the  network  of  the  sinusoidal 
spaces.  The  auricle,  Au,  is  without  sinusoids.  The  great  venous  trunk  or 
ductus  venosus,  D.  ven,  opens  into  the  auricle,  the  opening  being  guarded  by 
two  valves,  that  on  the  dorsal  side  of  the  opening  in  the  figure,  V.  s,  being  the 
left  valve.  The  ductus  venosus  receives  its  blood-supply  from  the  liver,  Li, 
which  consists  of  liver  cells  or  hepatic  cylinders  and  numerous  sinusoids  of  many 
diameters.  On  the  lower  side  of  the  liver  there  is  a  considerable  accumulation  of 
mesenchyma  by  which  the  liver  is  united  on  the  one  end  to  the  body- wall,  Som,  to 
the  umbilical  wall,  Um.  wf ,  and  to  the  mesentery  by  which  the  intestine  is  sus- 
pended from  the  liver.  In  this  mesenchyma  are  lodged  three  spaces  bounded 
by  entoderm,  the  uppermost  of  which  is  indicated  by  the  reference  line,  A .  It  is 
just  at  this  point  that  the  development  of  the  gall-bladder  and  the  intestine 
takes  place,  and  the  exact  identity  of  the  three  entodermal  structures  just  re- 
ferred to  has  not  yet  been  satisfactorily  worked  out.  Underneath  the  liver  in 
the  section  of  the  mesentery  is  situated  the  portal  vein,  P.  v.  From  the  mesen- 
tery extends  out  the  intestine  (duodenum).  It  is  a  somewhat  cylindrical  tube 
which  curves  over  ventral  wards  and  passes  out  through  the  opening  of  the  umbil- 
icus. It  consists  of  a  very  small  tube  of  entoderm,  Ent,  with  only  a  small  inter- 
nal cavity  (compare  Fig.  132,  Red.).  The  thickness  of  the  intestinal  wall  is  due 
chiefly  to  the  considerable  development  of  the  mesenchyma.  The  external 
covering  of  the  intestine  is  a  layer  of  mesothelium  which  becomes  the  peritoneal 
epithelium  of  the  adult.  In  the  tissue  of  the  organ  we  distinguish  the  narrow 
vitelline  artery,  Art.  v.  The  umbilical  opening  is  quite  wide  and  is  bounded  both 
above  and  below  by  a  prolongation,  Um.  wf,  Um.  w" ,  of  the  somatopleure  of  the 
embryo.  The  wall  on  the  upper  side  is  much  thicker  than  on  the  lower.  The 
opening  of  the  umbilicus  is  very  wide.  It  is  partly  occupied  by  the  duodenum. 
Appended  to  the  inferior  wall  of  .'.if  umbilu  :  is  the  allantois,  All,  which 
arises  from  the  enlarged  caudal  ^nd  (cloaca),  C  lot  of  the  intestine.  It  passes 


222  STUDY  OF  PIG  EMBRYOS. 

out  first  forward,  then  makes  an  acute  but  rounded  angle,  and  extends  outward 
through  the  umbilical  opening.  It  may,  therefore,  be  said  to  consist  of  two  limbs, 
one  within  the  body  of  the  embryo  joining  the  cloaca,  and  the  other  passing  out 
through  the  umbilical  opening.  The  limb  arising  from  the  cloaca  is  completely 
united  with  the  body-wall,  and  is,  of  course,  upon  the  side  toward  the  ccelom 
covered  in  by  mesothelium.  The  lining  of  the  allantoic  cavity  is  an  epithelium, 
and  is  a  portion  of  the  entoderm.  Along  the  second  limb  of  the  allantois  the 
mesothelium  on  the  side  toward  the  cavity  of  the  umbilicus  forms  a  series  of 
clumsy  projections,  Vil,  the  mesothelial  villi  of  the  allantois.  They  are  smallest 
toward  the  embryo  and  increase  in  size  distally.  With  higher  power  one  can 
see  that  the  mesothelium  of  the  villi  is  very  thin  and  the  mesenchyma  in  their 
interior  of  quite  loose  texture.  In  later  stages  the  mesothelium  grows,  the 
mesenchyma  in  large  part  disappears,  and  the  villi  then  seem  hardly  more  than 
small  bags  of  mesothelium  with  but  little  contents,  save  some  coagulum.  They 
continue  to  enlarge  until  the  embryo  is  1 7  or  1 8  mm.  long,  after  which  they  begin 
to  abort.  In  these  older  stages  the  villi  extend  far  into  the  abdomen  and  are 
packed  in  between  the  abdominal  viscera,  presenting  curious  appearances  in 
section.  As  the  tail  of  the  embryo  is  bent  to  one  side,  it  offers  us  a  section  of 
a  portion  of  the  spinal  cord,  Sp.  c,  and  at  its  tip  a  glimpse  of  three  primitive  seg- 
ments, Seg. 

Frontal  Section  through  the  Mid-brain  and  Fore-brain. — Comparison  with 
figure  99  (pig,  10  mm.)  will  make  it  clear  that  in  a  frontal  series  we  shall  obtain 
a  few  sections  of  the  head  which  include  only  mid-brain  and  fore-brain  and  show 
no  other  special  cephalic  structures.  The  mid-brain,  M.  B,  is  somewhat  rounded 
in  form  and  passes  over  into  the  fore-brain,  which  is  quite  long  and  which  already 
shows  traces  of  its  subdivision  into  two  parts,  the  diencephalon,  Dien,  wrhich  lies 
nearest  to  the  mid-brain,  and  the  prosencephalon,  Pros,  which  constitutes  the 
terminal  portion  of  the  brain  and  which  produces  the  lateral  expansions  which 
are  to  form  the  cerebral  hemispheres.  The  expanding  prosencephalon  is  sepa- 
rated by  a  constriction  from  the  diencephalon,  which  in  its  turn  is  similarly  sepa- 
rated from  the  mid-brain.  The  diencephalon  and  prosencephalon  together  rep- 
resent the  fore-brain.  They  are  subdivisions  of  the  primary  first  cerebral  vesi- 
cles. It  is  important  to  note  that  they  do  not  correspond  to  complete  subdivis 
ions,  and  have  not  the  same  morphological  value  as  the  three  primary  vesicles. 
The  histological  development  is  much  less  advanced  than  in  the  pig  of  12  mm. 
The  ectoderm  is-  very  thin,  consisting  for  the  most  part  of  a  single  layer  of  cells, 
but  here  and  there  the  formation  of  a  second  layer  is  seen  to  be  beginning.  The 
mesoderm  is  very  simple  in  ,  .  \  almost  uniform  in  appearance,  but 

there  is  a  distinct  difference  between  the  meseiuhyma  around  the  brain  and  that 


EMBRYO  OF  9  MM. 


223 


Md. 


M.B. 


Dien. 


underneath  the  epidermis,  the  former  having  cells  further  apart.  This  is  almost 
the  first  stage  in  the  differentiation  of  the  arachnoid  zone  around  the  brain.  The 
pia  mater,  however,  though  quite  thin,  is  well  defined,  by  the  condensation  of  the 
mesenchymal  cells  and  by  the  somewhat  numerous  small  blood-vessels  in  it. 
The  medullary  wall  is  everywhere  quite  thick  and  crowded  with  nuclei.  In  the 
region  of  the  diencephalon  the  ectoglia  is  distinctly  formed,  but  elsewhere  has 
hardly  begun  its  differentiation.  On  the  inside  of  the  medullary  wall,  close  to 
the  surface,  there  are  everywhere  very  numer- 
ous mitotic  figures. 

Frontal  Section  through  the  Umbilical  Open- 
ing.— The  illustration  (Fig.  132)  is  part  of  the 
same  section  in  the  series  from  which  figure  131 
is  taken.  For  convenience  of  comparison  the 
position  has  been  reversed  so  as  to  bring  the 
dorsal  side  of  the  embryo  uppermost  in  figure 
132.  It  results  from  this  that  the  right  and 
left  sides  of  the  embryo  are  reversed  in  the 
engraving  as  compared  with  the  other  figures 
of  transverse  and  frontal  sections.  By  exam- 
ining figure  99  (pig,  10  mm.)  the  student  will 
see  that  sections  in  the  frontal  plane,  owing  to 
the  curvature  of  the  posterior  end  of  the  body- 
wall,  furnish  transverse  sections  of  the  spinal 
cord  of  the  pelvic  region.  Therefore,  the  sec- 
tion here  figured,  although  part  of  a  frontal 
series,  is  directly  comparable  to  a  transverse 
section  of  the  body.  In  the  upper  part  of  the 
figure  we  have  the  spinal  cord,  Sp.  c,  and  on  one 
side  of  that  the  ganglion,  G.  Owing  to  the 
spiral  twist  of  the  embryo  the  section  is  not 
symmetrical,  so  that  the  posterior  limb,  P.  L, 

appears  only  on  one  side  of  the  section.  Laterad  from  the  nerve  shown  in 
the  figure  is  the  large  muscle  plate,  My,  the  cells  of  which  are  already  begin- 
ning to  change  into  muscle-fibers.  On  the  dorsal  side  of  the  plate  we  find 
its  growing  edge,  m.  pi,  where  the  tissue  of  the  muscle  plate  proper  bends 
over  and  passes  continuously  into  the  external  wall  of  the  segment.  From 
this  growing  edge  the  cells  are  added  to  the  muscle  plate  by  which  it  ex- 
tends upward.  The  similar  edge  on  the  ventral  side  provides  for  the  extension 
of  the  muscle  plate  downward.  In  the  median  line,  below  the  spinal  cord,  we 


Pros. 


FIG.    131. — PIG,   9.0   MM.      FRONTAL 

SERIES  54,  SECTION  194. 
Dien,  Diencephalon.      M.B,   Mid-brain. 
Md,  Medullary  wall  of  brain,     t/ies, 
Mesenchyma.    Pros,  Prosencephalon. 
Ve,  Vein.     X  22  diams. 


224 


STUDY  OF  PIG  EMBRYOS. 


have  the  small  notochord,  Nch,  and  the  large  median  dorsal  aorta,  Ao.  In  the 
ventral  portion  of  the  embryo  appears  the  large  body-cavity  into  which  protrude 
the  Wolffian  bodies  and  the  intestine.  The  ccelom  also  has  a  downward  prolon- 

tti.pl.     G.         Sp.c. 


V.U.S. 


In 


FIG.  132. — PIG,  9.0  MM.    FRONTAL  SERIES  54,  SECTION  194. 

Ao,  Aorta,  card,  Cardinal  vein.  ./%  Ectodermal  fold  at  the  border  of  the  limb-bud.  G,  Ganglion,  gen* 
Genital  ridge.  Glo,  Glomerulus.  In,  Small  intestine  (jejunum),  mes,  Splanchnic  mesoderm  (of  the 
intestinal  wall),  m.pl,  Dorsal  'growing  edge  of  the  muscle  plate.  My,  Muscle  plate  of  secondary 
segment.  Nch,  Notochord.  P.L,  Posterior  limb.  Rect,  Large  intestine.  Sow,  Somatopleure.  Sp.c, 
Spinal  cord.  V.U.D,  Right  umbilical  vein.  V.U.S,  Left  umbilical  vein.  W.b,  Wolffian  body. 
W.D,  Wolffian  duct.  X  35  diams. 


gation  into  the  beginning  of  the  umbilical  cord,  and  in  this  prolongation  lies  the 
loop  of  the  intestine,  In.     The  coelom  is  bounded  everywhere  by  the  layer  of 


EMBRYO  OF  p  MM.  225 

mesothelium  represented  in  the  engraving  as  a  continuous  line.  With  a  higher 
power  the  mesothelium  is  seen  to  consist  of  a  single  layer  of  cells,  but  varying 
somewhat  in  thickness  in  different  regions.  By  following  the  contour  of  the 
mesothelium  the  student  will  recognize  at  once  that  all  of  the  viscera  are,  in  the 
anatomical  sense,  outside  of  the  crelom.  The  Wolffian  bodies,  W.  b,  are  vol- 
uminous organs  projecting  from  below  the  aorta  on  either  side  of  the  large  intes- 
tine, Red,  and  extending  far  into  the  abdominal  cavity.  At  the  lower  ventral 
edge  of  the  Wolffian  body  appears  the  Wolffian  duct,  W.  D,  a  wide,  longitu- 
dinal canal  into  which  the  Wolffian  tubules  open.  The  large  size  of  the  duct  is 
characteristic  of  this  stage.  In  later  stages  it  is  smaller.  The  tubules  are  very 
large,  contorted  in  their  course,  and  appear,  therefore,  variously  cut.  They  are 
formed  by  a  cuboidal  epithelium  and  are  provided  with  a  sinusoidal  circulation. 
The  endothelium  of  the  blood  spaces  can  generally  be  seen  fitting  closely  against 
the  epithelium  of  the  tubules.  Here  and  there,  however,  there  is  some  mesen- 
chyma  between  the  blood  spaces  and  the  walls  of  the  tubules.  On  the  median 
side  of  the  Wolffian  body  are  the  glomeruli,  which  are  of  large  size,  and  similar 
in  structure  to  the  glomerulus  of  the  permanent  kidney,  though  differing  from  the 
renal  glomeruli  in  their  proportions  and  in  the  details  of  their  structure.  It  is  not 
difficult  to  make  a  reconstruction  of  the  course  of  a  single  tubule  by  following  it 
through  a  few  neighboring  sections.  The  general  course  of  a  tubule  is  in  the 
transverse  plane,  but  it  is  much  contorted.  Bach  tubule  begins  at  one  of  the 
glomeruli,  with  which  it  is  in  open  communication.  It  then  bends  so  as  to  make 
a  somewhat  irregular  S-shaped  figure,  and  finally  opens  into  the  Wolffian  duct. 
After  leaving  the  glomerulus  it  widens  somewhat,  but  before  it  joins  the  Wolffian 
duct  it  again  diminishes  in  diameter.  The  changes  in  diameter  are  gradual. 
The  blood  spaces  or  sinusoids  of  the  Wolffian  body  are  derived  from  the  posterior 
cardinal  veins.  The  veins  and  tubules,  when  the  latter  first  become  distinct,  lie 
near  together.  As  development  continues  both  enlarge  and  encroach  upon  one 
another's  territory;  hence  there  is  an  intimate  intercrescence  of  the  blood-vessels 
and  of  the  tubules,  resulting  in  the  formation  of  sinusoids.  The  whole  of  the 
Wolffian  body  might  from  one  point  of  view,  therefore,  be  regarded  as  a  modifi- 
cation of  the  cardinal  vein,  and  morphologically  all  of  the  blood  spaces  between 
the  tubules  belong  to  that  vein.  There  remain  typically  two  portions  of  the 
cardinal  vein  which  are  more  or  less  open  and  distinct.  The  one  on  the  dorsal 
side  of  the  Wolffian  body,  car d,  may  be  conveniently  regarded  as  representing 
the  original  cardinal  vein.  The  other,  on  the  ventral  side  of  the  Wolffian  body,  is 
at  first  not  a  very  distinct  channel,  but  gradually  becomes  more  and  more  so,  and 
is  known  by  the  distinctive  name  of  subcardinal  vein.  It  is  a  vessel  of  great  mor- 
phological importance,  since  on  the  right  side  of  the  embryo  it  acquires  a  connec- 
15 


226  STUDY  OF  PIG  EMBRYOS. 

tion  with  the  liver  which  renders  it  possible  for  the  blood  of  the  right  subcardinal 
vein  to  pass  through  the  blood  spaces  of  the  liver  directly  to  the  heart.  This 
makes  a  very  direct  channel,  a  more  direct  one  than  existed  previously,  when  the 
blood  from  the  subcardinal  came  to  join  that  of  the  cardinal,  passing  up  to  the 
duct  of  Cuvier  and  then  back  to  the  heart.  The  new  channel  through  the  liver 
rapidly  enlarges  and  becomes  recognizable  as  the  vena  cava  inferior.  This  im- 
portant venous  trunk  is  a  combined  vessel,  comprising,  first,  a  part  of  the  sinus 
venosus  of  the  heart;  second,  the  ductus  venosus  of  the  liver;  third,  a  large 
channel  developed  from  the  sinusoids  of  the  liver;  fourth,  the  upper  part  of  the 
right  subcardinal  vein;  and,  fifth,  the  lower  part  of  the  right  cardinal.  The  vena 
cava  inferior  has  already  been  developed  in  the  pig  embryo  of  9  mm.  Be- 
tween the  two  Wolffian  bodies  hangs  down  the  large  intestine,  Rect,  suspended 
by  its  mesentery  in  the  median  line.  The  entodermal  portion  is  a  very  small 
circle  of  epithelium  with  an  extremely  minute  lumen,  which  in  the  section  is 
scarcely  larger  than  a  single  nucleus.  The  mesentery  and  intestine  are  covered 
by  a  well-defined  mesothelium  and  have  a  considerable  amount  of  mesenchyma, 
in  which  there  is  no  distinct  histological  differentiation  beyond  the  presence  of  a 
number  of  small  blood-vessels.  At  this  stage  the  large  intestine  runs  nearly  in 
the  median  plane  to  the  pelvic  end  of  the  body.  In  the  opposite  direction,  to- 
ward the  head,  it  bends  to  the  left  of  the  embryo,  making  a  loop  which  passes  over 
into  the  end  of  the  ileum,  The  ileum  forms  the  continuation  of  the  loop  and  ex- 
tends into  the  ccelom  at  the  base  of  the  umbilical  cord.  There  it  bends  back  and 
returns  toward  the  dorsal  side  of  the  embryo  to  pass  over  into  the  duodenum  and 
join  the  stomach.  Owing  to  the  fact  that  the  small  intestine  extends  into  the 
extra-embryonic  ccelom  of  the  umbilical  cord,  there  makes  a  loop,  and  returns  to 
the  embryonic  region,  we  get  typically  a  double  section  of  the  intestine  as  shown 
in  the  figure,  one  of  each  limb  of  the  loop.  The  entoderm,  In,  in  these  loops  forms 
a  small  ring,  which,  however,  is  much  larger  than  the  entodermal  ring  of  the  large 
intestine  at  this  stage.  Each  loop  contains  a  large  amount  of  mesenchyma,  mes, 
the  cells  of  which  are  somewhat  crowded,  so  that  the  tissue  appears  dark  in  the 
stained  section.  The  boundary  between  the  body  of  the  embryo  and  the  tissue 
of  the  umbilical  cord  is  marked  by  the  position  of  the  two  umbilical  veins,  that  of 
the  left  side,  V.  U.  S,  being  very  much  larger  than  that  of  the  right  side,  V.  U.  D. 
By  following  down  the  somatopleure,  Som,  of  the  embryo,  it  will  be  seen  that 
these  veins  are  lodged  therein,  and  that  the  continuation  of  the  somatopleure 
beyond  these  veins  forms  the  substance  of  the  umbilical  cord.  The  limb-bud, 
P.  L,  is  a  large  mass  of  rather  dense  mesenchyma,  entirely  without  muscles  or 
nerves  and  covered  by  ectoderm.  At  the  edge  of  the  limb-bud  the  ectoderm 
shows  a  special  thickening,  F.  The  theory  has  been  advanced  that  this  thicken- 


EMBRYO  OF  9  MM.  227 

ing  is  homologous  with  the  ectodermal  fold  which  produces  the  fin  of  fishes,  or 
at  least  that  portion  of  the  fin  in  which  the  fin-rays  are  developed. 

Frontal  Section  through  the  Second  and  Third  Gill  Clefts. — In  this  pre- 
paration (Fig.  133)  the  section  hits  the  posterior  wall,  Ot,  of  the  otocyst 
and  is  just  anterior  to  the  origin  of  the  glossopharyngeal  nerve.  The  appear- 
ance of  the  section  of  the  hind-brain  is  characteristic  for  this  region  of  young 
embryos.  The  deck-plate  has  grown  gradually  in  size  and  forms  a  wide  mem- 
brane, epen,  the  ependymal  roof  of  the  fourth  ventricle.  Owing  to  this  growth 
of  the  deck-plate,  the  upper  or  dorsal  limits  of  the  dorsal  zones,  D.  Z,  are  brought 
far  apart  and  the  cavity  of  the  hind-brain  is  thus  enlarged.  The  dorsal  zone  is 
marked  by  an  angle  in  the  interior  and  by  the  point  of  entrance  of  the  nerve- 
roots  on  the  exterior  from  the  ventral  zone,  V.  Z.  On  their  dorsal  side  the  dorsal 
zones  thin  out  and  pass  over  gradually  into  the  ependyma.  The  ependyma  con- 
sists of  a  single  layer  of  cells.  In  the  dorsal  zone  the  differentiation  of  the  three 
primary  layers  of  the  medullary  wall  has  scarcely  begun,  but  in  the  ventral  zones 
the  three  layers  are  already  distinguishable,  though  not  far  advanced  in  their 
differentiation.  In  the  floor-plate  there  are  two  layers.  Below  the  medullary 
tube  lies  the  basilar  artery,  A.  has,  and  below  that,  not  far  from  the  upper  wall 
of  the  pharynx,  lies  the  small  round  notochord  in  the  midst  of  loose  mesenchymal 
cells,  which  have  not  yet  begun  to  condense  themselves  about  the  notochord. 
The  pharynx  is  a  wide  space  of  rather  small  dorso-ventral  diameter,  and  having  a 
much  thinner  layer  of  entoderm  on  its  dorsal  than  on  its  ventral  side.  Above 
the  pharynx  on  either  side  lies  the  section  of  the  descending  aorta,  Ao.  d.  The 
reference  line  to  this  vessel  crosses  a  dark  mass  of  cells  which  belong  to  the  gan- 
glion nodosum  of  the  ninth  nerve.  Below  the  pharynx  the  section  shows  the 
third  aortic  arch,  Ao.  j,  and  the  fourth  aortic  arch,  Ao.  4.,  just  springing  off  from 
the  median  aortic  trunk  above  the  heart,  so  that  the  two  fourth  arches  are 
connected  across  the  median  line.  Between  the  third  and  fourth  aortic  arches 
on  either  side  is  a  small  cavity  lined  by  entoderm,  cl.  Ill,  a  diverticulum  from 
the  third  gill  cleft.  Immediately  below  the  otocyst  is  the  jugular  vein,  Jug. 
From  a  point  below  the  jugular  there  extends  a  prolongation,  Hy,  which  may  be 
taken  as  a  portion  of  the  hyoid  or  second  branchial  arch.  It  extends  downward 
and  consists  of  a  mass  of  mesenchyma  covered  by  ectoderm.  It  encloses  a  space, 
cl.  II.  ex,  which  may  be  regarded  as  the  external  portion  of  the  second  gill  cleft. 
In  a  neighboring  section  (455)  the  prolongation  of  the  pharynx  shown  in  figure 
133  can  be  traced  still  further  until  it  opens  into  this  space,  cl.  II.  ex.  The  second 
cleft  is  open  upon  both  sides  of  the  embryo,  the  first  and  third  have  closing  mem- 
branes, the  fourth  cleft  is  not  yet  so  far  developed  that  its  entoderm  has  come  in 
contact  with  the  epidermis  of  the  embryo.  The  second  cleft  probably  always 


228 


STUDY  OF  PIG  EMBRYOS. 


becomes  open,  differing  in  this  respect  from  all  the  others.  Why  it  has  this  pecu- 
liarity we  do  not  know.  The  opening  does  not  persist,  but  the  exact  history  of 
its  closure  is  at  present  unknown.  The  process,  Hy,  described  as  shutting  in  the 
external  portion  of  the  second  gill  cleft  has  sometimes  been  termed  the  opercu- 
lum,  because  it  covers  a  gill  cleft  opening,  as  does  the  operculum  of  a  bony  fish. 


EC.  mes.     Epen. 


D.Z. 


FIG    133. — PIG,  9.0  MM.     FRONTAL  SERIES  54,  SECTION  459. 

A. bus,  Basilar  artery.  Ao.d,  Descending  aorta  of  the  left  side.  Ao. j,  Third  aortic  arch.  Ao.j.,  Fourth 
aortic  arch  arising  from  the  median  ventral  aorta.  cl.II.ex,  External  portion  of  the  second  gill  cleft. 
cl.III,  Third  gill  cleft.  D.Z,  Dorsal  zone  of  the  medulla  oblongata.  EC,  Ectoderm.  Epen,  Ependymal 
roof  of  the  hind-brain.  Hy,  Hyoid  arch.  Jug,  Jugular  vein,  mes,  Mesenchyma.  Ot,  Posterior  wall 
of  the  otocyst.  P.Ao,  Pulmonary  aorta.  Ph,  Pharnyx.  •  Som,  Somatopleure.  V.Z,  Ventral  zone. 
X  22  diams. 

In  the  lower  part  of  our  figure  a  portion  of  the  somatopleure,  Som,  is  shown  where 
it  extends  ventralwards  to  form  the  wall  of  the  pericardial  cavity.  There  is  also 
included  in  the  drawing  a  part  of  the  pulmonary  aorta,  P.  Ao. 

Pig  Embryo  of  6  mm. 

Of  this  stage  two  transverse  sections  are  figured  in  order  to  give  more  exact 
notions  as  to  the  structure  of  neuromeres  and  the  secondary  segments.  The  first 
section  is  taken  through  the  level  of  the  head,  and  may  be  directly  compared  with 
figure  113.  The  relations  are  so  closely  similar  that  it  is  unnecessary  to  describe 
the  present  section  (Fig.  134)  in  detail.  The  explanation  of  the  figure  is  suffi- 


EMBRYO  OF  6  MM. 


229 


cient  for  the  identification  of  the  parts.  The  otocyst  is  large  and  conspicuous, 
and  the  arrangement  of  the  nerves  is  essentially  similar  to  what  we  find  in  the 
older  embryos.  The  neuromeres,  however,  are  very  distinct,  especially  those 


Vtn.ll' 


A. car. 


Yen.  III.        Md. 

FIG.  134. — PIG,  6.0  MM.     TRANSVERSE  SERIES  9,  SECTION  90. 

A. car,  Carotid  artery.  Jug,  Jug' ,  Jugular  vein.  Md,  Medullary  wall  of  the  fore-brain.  N.I,  N.2,  N.j,  N.j, 
Neuromeres  of  the  hind-brain.  N.j,  Trigeminal  ganglion.  N.jfi,  Acustico-facial  ganglion.  N.Q,  Root 
of  the  glosso-pharyngeal  nerve.  Of,  Otocyst.  Ven.ll  I,  Third  ventricle  or  cavity  of  the  fore-brain. 
Ven.IV,  Fourth  ventricle.  X  35  diams. 

upon  the  left  side  of  the  embryo,  N.  i,  2,3,  4.     Of  these,  the  third  is  perhaps 
the  most  characteristic.     Each  neuromere  is  separated  from  its  fellow  by  an 


230 


STUDY  OF  PIG  EMBRYOS. 


A.  is. 


internal  sharp  ridge,  so  that  the  inner  boundary  of  each  neuromere  toward  the 
cavity  of  the  fourth  ventricle  is  a  small  arc  of  a  circle.  The  cells  are  elongated 
and  are  placed  radially  to  the  inner  curved  surface  of  the  neuromere.  A  thin  but 
distinct  layer  of  ectoglia  is  present.  The  light  line,  which  marks  the  boundary 

between  the  adjacent  neuromeres,  is 
produced'  by  the  comparative  absence 
of  nuclei.  As  to  the  number  of  neuro- 
meres our  knowledge  is  still  defective; 
nor  have  we  yet  succeeded  in  making 
sure  of  their  exact  relation  to  the  nerves 
of  the  head,  though  such  a  relation  evi- 
dently exists.  Thus  we  find,  for  exam- 
ple, that  the  facial  nerve  is  always  con- 
nected with  neuromere  2  of  our  figure, 
and  the  glosso-pharyngeal  nerve  with 
neuromere  4. 

Our  second  section  is  very  near  the 
end  of  the  same  series.  Owing  to  the 
curvature  of  the  posterior  end  of  the 
body  of  younger  embryos  (compare  Fig. 
99;  pig  10  mm.),  sections  taken  in  the 
plane  which  we  call  transverse  strike 
the  lumbar  region  so  as  to  give  longi- 
tudinal sections  of  the  spinal  cord 
and  primitive  segments.  Figure  135, 
therefore,  shows  the  cavity  of  the 
spinal  cord,  Sp.  c,  cut  for  a  very  long 
distance.  At  the  upper  and  lower  ends 
of  the  section  the  dorsal  side  of  the 
spinal  cord  is  cut,  and  accordingly  we 
see  at  these  levels  sections  of  the  gan- 
glia, G,  on  either  side  of  the  spinal  cord. 
In  the  middle  of  our  section  the  ven- 
tral portion  of  the  spinal  cord  is  cut, 
and  here,  therefore,  the  ventral  roots, 

V.  R,  of  the  nerves  are  displayed.  The  segments  are  clearly  marked  by  the 
external  configuration  of  the  embryo,  the  ectoderm,  EC,  forming  an  arch  over 
the  outside  of  each  segment.  Each  mesodermic  segment  shows  three  distinct 
parts  :  next  to  the  ectoderm  the  broad,  epithelioid  cutis  plate,  within  which 


FIG.  135. — PIG,  6.0  MM.  TRANSVERSE  SERIES  9, 
SECTION  519. 

A. is,  Intersegmental  artery.  Cu,  Cutis  plate.  EC, 
Ectoderm.  G,  Ganglion.  muse,  Muscle 
plate.  ScZer,  Sclerotome,  auct.  Sp.c,  Spinal 
cord.  V.R,  Ventral  nerve-root.  X  5°  diams. 


EMBRYO  OF  17  MM.  231 

comes  the  spindle-shaped  section  of  the  inner  portion  of  the  segment,  muse, 
the  anlage  of  the  skeletal  muscles ;  and,  third,  an  expanding  mass  of  mesenchyma, 
Scler,  which  is  sometimes  termed  the  sclerotome.  This  term,  however,  is  not 
wholly  felicitous,  because  this  mesenchyma  forms  not  only  the  segments  of  the 
skeleton,  but  the  connective  tissue  of  the  whole  region  about  the  spinal  cord  in  the 
dorsal  part  of  the  embryo.  The  figure  shows  very  clearly  that  the  ganglia  and 
ventral  nerve-roots  are  arranged  in  exact  conformity  to  the  segments,  and  it  can 
be  easily  observed,  by  following  through  the  series  of  sections,  that  for  each 
segment  there  is  one  ganglion  and  one  ventral  root.  It  also  shows  that  the  ventral 
roots  reach  directly  to  the  muscle  plate.  The  muscle  plate  is  histologically 
partly  differentiated,  for  its  cells  have  already  elongated  in  a  direction  parallel 
with  the  longitudinal  axis  of  the  embryo,  and  their  nuclei  also  have  become  much 
larger  than  any  other  nuclei  in  the  neighboring  parts  of  the  embryo,  being  per- 
haps three  times  as  large  as  the  mesenchymal  nuclei  of  the  sclerotome.  They 
are  oval  in  form,  contain  many  fine,  and  usually  one  or  two  somewhat  larger 
granules,  the  larger  ones  being  deeply  stained,  but  the  nuclei,  as  a  whole,  are 
stained  more  lightly  than  their  neighbors.  Each  segment  is  very  clearly  sepa- 
rated from  its  neighbors,  and  between  the  ends  of  the  adjacent  muscle  plates  there 
is  a  small,  clear  space  entirely  free  from  cells  and  extending  outward  to  the  epi- 
dermis. Just  inside  of  this  space  in  every  case  is  a  small  blood-vessel,  the  inter- 
segmental  artery,  A .  is.  The  intersegmental  arteries  are  small  branches  which 
arise  in  symmetrical  pairs  from  the  dorsal  aorta. 

Pig  Embryo  of  17  mm. 

Since  the  pig  of  12  mm.  contains  the  anlages  of  perhaps  every  important 
part  of  the  body,  sufficiently  advanced  in  development  to  be  clearly  recognized, 
we  find  in  the  immediate  subsequent  development  that  we  have  to  do  not  so 
much  with  an  introduction  of  new  parts  as  with  the  differentiation  of  those  which 
have  already  commenced.  Embryos  of  17  mm.  are  convenient  for  the  study  of 
the  differentiations  referred  to.  Particularly  important  for  the  student  to  note 
are  the  advances  in  the  development  of  the  vertebrae,  of  the  lungs,  of  the  Wolffian 
bodies  and  genital  glands,  and  of  the  kidneys.  These  points  are  illustrated  in 
figures  136  to  138,  representing  portions  of  three  transverse  sections  of  a  17  mm. 
embryo. 

Transverse  Section  through  the  Lungs. — The  epidermis  of  the  embryo  has 
become  more  distinct  owing  to  its  growth  in  thickness,  which  is  accomplished  by 
the  increase  of  the  number  of  layers  of  cells.  The  growth  is  very  marked  at  the 
sides  of  the  section  about  the  level  of  the  vertebra.  At  these  points  it  can  be 
clearly  seen  that  upon  the  outside  the  epidermis  has  a  very  thin  layer  of  flattened 


232 


STUDY  OF  PIG  EMBRYOS. 


cells,  the  nuclei  of  which  are  themselves  also  somewhat  flattened.  This  single 
layer  of  cells  is  known  as  the  epitrichium,  because  the  hairs  are  developed  en- 
tirely underneath  it.  Where  the  epidermis  is  thickest,  one  can  observe  that  the 
layers  of  cells  next  to  the  mesoderm  are  closely  packed  together  with  round 
.nuclei.  They  represent  the  commencing  formation  of  the  basal  layer  of  the  adult 
epidermis.  Between  the  basal  layer  and  the  epitrichium  the  cells  are  more 


Vert. 


D.R.         Ec.gl.       Sp.c.        fin.  G. 


Nek. 


Ve'.  • 


Ve." 


card. 


<,  FIG.  136. ~Pic,  17.0  MM.     TRANSVERSE  SERIES  51,  SECTION  464. 

Ao,  Aorta,  bro,  Entodermal  bronchus,  card,  Posterior  cardinal  vein,  cin,  Neurone  layer  (cinerea)  of  spinal 
cord.  Cost,  Anlage  of  ribs.  D.R,  Dorsal  root.  Ec.gl,  Ectoglia.  G,  Ganglion.  Li,  Liver.  Lu, 
Lung,  muse,  Dorsal  musculature.  N.io,  Vagus  nerve.  Nch,  Notochord.  CE,  (Esophagus.  PI. ecu, 
Pleural  ccelom.  R.D,  Ranaus  dorsalis.  R.  V,  Ramus  ventralis.  R.sy,  Ramus  sympathicus.  Sp.c, 
Spinal  cord.  Sym,  Sympathetic  ganglion.  Vef ,  Ve" ,  Branches  of  the  subclavian  vein.  Vert,  Vertebra. 
X  22  diams. 

loosely  placed,  forming  the  initial  stage  of  the  mucous  layer.  The  mesenchyma 
is  very  much  developed  and  occupies  a  large  territory  in  the  dorsal  region  of  the 
embryo.  It  carries  the  nerves  and  blood-vessels  and  shows  at  various  points 
accumulations  of  more  darkly  stained  cells,  which  are  of  two  kinds :  first,  groups 
of  mesenchymal  cells  proper,  the  anlages  of  portions  of  the  skeleton ;  and,  second, 


EMBRYO  OF  17  MM.  233 

groups  of  mesothelial  muscle  cells,  the  anlages  of  the  various  skeletal  muscles. 
There  is  little  differentiation  otherwise  in  the  mesenchyma,  but  we  may  note  the 
following  changes  in  it :  (i)  The  anlage  of  the  vertebra,  Vert,  which  is  now  quite 
well  defined ;  around  the  edge  of  it  the  cells  have  assumed  an  elongated  form  and 
have  elongated  nuclei;  the  elongation  is  parallel  with  the  surface  of  the  anlage. 
These  cells  result  from  the  commencing  differentiation  of  the  perichondrium, 
which  at  this  stage  merges  on  the  one  side  into  the  anlage  of  the  vertebrae,  and  on 
the  other  into  the  surrounding  mesenchyma.  The  cells  of  the  vertebra  have 
changed  into  young  cartilage  cells.  They  are  now  distinctly  separated  from  one 
another  by  a  well-developed  matrix.  Each  cell  occupies  a  separate  space  or 
capsule  in  the  matrix.  The  protoplasm  of  the  cell  having  almost  completely 
disappeared,  only  the  nucleus  remains  distinct.  It  stains  readily,  has  a  distinct 
outline,  and  contains  a  number  of  dark  granules,  one  or  two  of  which  are  conspicu- 
ous by  their  greater  size  and  irregular  shape.  The  nucleus  itself,  in  most  of  the 
cells,  is  somewhat  irregular  in  outline,  as  if  distorted  by  shrinkage.  Toward  the 
center  of  the  anlage  the  cytomorphosis  is  most  advanced.  Toward  its  outer  sur- 
face the  cells  are  less  changed,  lie  nearer  together,  and  have  more  regularly 
shaped  nuclei.  In  the  center  of  the  vertebra  lies  the  round  notochord,  A/c&,the 
sheath  of  which  has  increased  considerably  in  thickness,  and,  being  unstained, 
appears  as  a  clear  space  between  the  cells  of  the  notochord  and  those  of  the  en- 
closing vertebra.  The  nuclei  in  the  notochord  are  numerous  and  somewhat 
crowded  together.  (2)  The  costal  processes,  Cost,  of  the  vertebra,  which  are 
rod-like  and  extend  quite  far  down  into  the  somatopleure.  The  histogenetic 
changes  in  these  processes  are  similar  to  those  in  the  vertebra,  but  less  advanced. 
They  have  progressed  somewhat  more  in  the  proximal  than  in  the  distal  portion 
of  the  rib.  (3)  Around  the  central  nervous  system  the  pia  mater  has  become  more 
distinct,  and  the  arachnoid  membrane  is  indicated  by  the  wide  separation  of  its 
cells  and  the  length  of  the  processes  connecting  them.  Its  differentiation  is  most 
easily  recognized  at  the  sides  of  the  spinal  cord.  The  outer  limit  of  the  arachnoid 
is  shown  by  a  slight  condensation  of  the  mesenchyma  which  marks  the  first  step 
in  the  differentiation  of  the  dura  mater,  the  anlage  of  which  is  further  defined 
by  the  elongated  form  of  the  mesenchymal  cells,  by  which  they  differ  from  the 
mesenchymal  cells  on  both  sides.  (4)  There  is  a  distinct  layer  of  condensed 
mesenchyma  around  the  aorta,  Ao.  The  layer  thus  formed  consists  of  elongated 
cells,  and  perhaps  corresponds  only  to  the  muscular  coat  of  the  vessel.  (5) 
About  the  oesophagus,  (E,  the  mesenchyma  forms  two  distinct  layers.  The 
inner,  next  to  the  epithelium,  is  of  looser  texture,  and  is  the  anlage  of  both  the 
mucous  and  submucous  layers  of  the  adult.  The  outer  layer  is  denser  and  con- 
sists chiefly  of  young  smooth  muscle  cells,  which  are  merely  modified  mesen- 


234  STUDY  OF  PIG  EMBRYOS. 

chymal  cells,  characterized  by  the  greater  development  of  their  protoplasm  and 
by  their  elongated  form.  Traces  of  the  differentiation  of  the  outer  layer  into 
the  inner  circular  muscular  coat  and  the  outer  longitudinal  coat  of  the  adult  are 
clear  in  the  section.  The  spinal  cord,  Sp.  c,  has  changed  its  outline  in  section, 
being  broadest  in  the  ventral  zones,  which  have  also  begun  to  expand  ventral- 
wards  so  that  the  outline  of  the  cord  shows  on  its  inferior  side  a  concavity,  the 
first  indication  of  the  ventral  fissure.  The  three  layers  of  the  spinal  cord  are 
very  distinct.  The  change  in  form,  however,  it  can  be  clearly  seen,  is  due 
chiefly  to  the  growth  of  the  gray  layer,  tin,  especially  in  the  ventral  zone.  The 
gray  layer  in  the  dorsal  zone  is  still  very  slightly  developed.  From  the  dorsal 
zone  descends  on  either  side  the  dorsal  nerve-root,  D.  R,  which  presently  joins 
the  ganglion,  G.  The  ganglion  now  occupies  a  much  lower  position  than  in  the 
earlier  stages  (compare  Fig.  122,  G).  From  the  ventral  zone  springs  the  ventral 
root  which  unites  with  the  dorsal  at  the  lower  tip  of  the  ganglion.  From  the 
nerve-trunk  thus  formed  there  is  given  off  almost  immediately  the  dorsal  branch , 
R.  D,  which  soon  ramifies  in  the  midst  of  a  dark  mass  of  tissue,  the  anlage  of  the 
dorsal  musculature.  The  main  nerve-trunk  descends  ventralwards  and  sends  off 
at  the  level  of  the  vertebra  a  sympathetic  branch,  R.  sym,  which  runs  obliquely 
downward  and  inward  toward  the  aorta,  and  there  terminates  in  the  anlage  of  the 
sympathetic  chain,  Sym,  which  consists  partly  of  nerve-fibers,  partly  of  ganglion 
cells  which  have  migrated  along  the  nerve  and  taken  up  their  position  at  its  end. 
These  cells  are  easily  recognized  by  their  very  dark  staining.  Their  nuclei  are  a 
little  lighter  than  those  of  the  neighboring  mesenchymal  cells,  but,  owing  to  their 
deep  coloration,  stand  out  conspicuously,  even  when  the  section  is  examined 
only  with  the  low  power.  The  sympathetic  anlage  comes  in  close  contact  with  a 
portion  of  the  cardinal  vein,  card,  near  the  aorta.  The  main  nerve-trunk,  R.  V, 
continues  obliquely  downward  and  presently  forks  into  an  upper  and  a  lower 
branch.  The  cardinal  veins,  card,  lie  on  either  side  of  the  aorta,  but  they  are 
almost  completely  obliterated  by  the  ingrowth  of  the  Wolffian  tubules,  which 
subdivide  the  vein  into  numerous  smaller  channels  or  sinusoids.  The  section 
also  shows  two  branches,  Vef,  and  Ve",  of  the  subclavian  vein.  The  identity  of 
these  branches  has  not  yet  been  determined.  The  aorta,  Ao,  is  a  very  large  ves- 
sel a  little  to  the  left  of  the  median  plane.  It  has  a  well-developed  muscular 
coat.  Beneath  it  follows  the  oesophagus,  the  lumen  of  which  is  much  smaller 
than  that  of  the  aorta.  Its  epithelium  has  the  general  characteristics  of  the 
epithelial  entoderm  at  this  stage,  being  a  rather  thick  cylinder  epithelium.  As 
above  mentioned,  the  differentiation  of  the  mucous  and  muscular  layers  of  the 
oesophagus  shows  clearly.  Below  the  oesophagus  lie  the  two  large  vagus  nerves, 
N.  10,  and  then  follow  the  sections  of  the  two  lungs,  Lu.  Each  lung  is  a  lobe  of 


EMBRYO  OF  17  MM.  235 

tissue  connected  with  its  fellow  across  the  median  line  of  the  embryo  and  pro- 
jecting laterally  far  into  the  pleural  cavity,  PL  cos.  Each  lung  consists  chiefly 
of  a  large  accumulation  of  dense  mesenchyma  in  which  the  epithelial  tfronchi, 
bro,  ramify.  Each  bronchus  has  a  central  lumen  and  its  walls  are  formed  by  a 
moderately  thick  layer  of  cylinder  entodermal  cells.  The  surface  of  each  lung  is 
covered  by  mesothelium,  which  is  shown  as  a  distinct  line  in  the  engraving.  The 
mesothelium  can  be  followed  to  the  root  of  the  lung,  where  it  is  reflected  on  to 
the  outer  wall  of  the  pleural  chamber.  The  pleural  cavity,  PI.  cos,  is  thus  every- 
where bounded  by  mesothelium  which  persists  throughout  life,  being  known  in 
the  adult  as  the  pleural  epithelium. 

Section  through  the  Wolffian  Body  and  Genital  Gland. — The  general  charac- 
teristics of  the  ectoderm,  mesenchyma,  and  nervous  system  are  nearly  the  same 
as  in  the  section  last  described.  On  one  side  the  section  shows  a  thickening  of 
the  ectoderm,  the  anlage  of  a  mammary  gland,  mam  (compare  page  249).  The 
branches  of  the  nerves  are  not  so  well  shown  in  this  section  as  in  the  previous  one. 
The  level  of  our  section  corresponds  to  the  lower  end  of  the  vena  cava  inferior, 
which  is  marked  at  this  stage  by  the  two  large  mesonephric  veins,  V.  msn,  which 
come  from  the  Wolffian  bodies  and  by  their  union  constitute  the  lower  end  of  the 
vena  cava.  The  mesonephric  veins  are,  strictly  speaking,  portions  thereof.  The 
Wolifian  bodies  are  the  most  conspicuous  structures  shown  in  the  section.  They 
consist  chiefly  of  a  great  number  of  tubules,  W.t,  very  much  crowded  together. 
On  the  median  side  of  the  organ  appear  the  large  glomeruli,  Glo,  and  on  their  ven- 
tral side  we  have  the  section  of  the  longitudinal  Wolffian  duct.  The  tubules  of 
the  Wolffian  body  are  formed  by  a  more  or  less  nearly  cuboidal  epithelium,  the 
nuclei  of  which  are  decidedly  larger  than  those  of  the  mesenchymal  cells.  The 
nuclei  themselves  stain  deeply,  have  well-marked  outlines,  and  very  distinct 
granules  in  their  interior.  The  protoplasm  of  the  cells  also  stains  somewhat  with 
cochineal,  carmine,  hematoxylin,  etc.  There  is  very  little  mesenchyma  in  the 
organ,  but  each  tubule  is  closely  invested  by  vascular  endothelium;  hence  the 
tubules  are  separated  from  one  another  only  by  blood  spaces,  which,  morphologi- 
cally speaking,  are  portions  of  the  cavity  of  the  cardinal  vein.  These  blood  spaces 
are  highly  characteristic  and  are  typical  sinusoids.  The  intertubular  circulation 
of  the  Wolffian  body  is,  so  far  as  known,  always. sinusoidal.  The  aorta,  Ao,  is 
seen  in  the  figure  to  give  off  a  small  branch,  art,  which  runs  toward  the  Wolffian 
body.  There  are  numerous  such  branches,  each  one  of  which  may  be  traced  to  a 
glomerulus  of  the  mesonephros.  Each  glomerulus  has  a  capillary  circulation, 
and  the  blood  on  leaving  the  glomerulus  is  supposed  to  be  emptied  into  the 
venous  sinusoids.  More  exact  investigation  of  this  point  is  needed.  The  meso- 
nephros is  covered  by  a  layer  of  mesothelium,  msth,  underneath  which  is  a  thin 


236 


STUDY  OF  PIG  EMBRYOS. 


layer  of  mesenchyma.  The  two  together  constitute  the  anlage  of  the  peritoneal 
covering  of  the  organ.  To  the  median  side  of  the  Wolffian  body  is  appended  the 
large  anlage  of  the  genital  gland,  Gen,  which  has  a  constricted  connection  with 
the  Wolffian  body.  Each  gland  is  covered  by  mesothelium  and  extends  until 


Nch. 


ec.gl. 


R.V.' 


R.sy. 


Ao. 


art. 


nisth . 


W.D.  In.     ,  Li.         nisi. 

FIG.  137. — PIG,  17.0  MM.     TRANSVERSE  SERIES  51,  SECTION  651. 

Ao,  Dorsal  aorta,  art,  Glomerular  artery.  Cce,  Coelom.  ec.gl,  Ectoglia.  G,  Ganglion.  Gen,  Genital 
gland.  Glo,  Glomerulus  of  Wolffian  body.  In,  Intestine.  Li,  Liver,  mam,  Mammary  anlage.  mst,  , 
Mesentery,  msth,  Mesothelium.  N,  Ventral  nerve.  Nch,  Notochord.  R.sy,  Ramus  sympathicus  of 
nerve.  R.  V ',  R.  V" ' ,  Branches  of  the  ventral  ramus  of  the  spinal  nerve.  Som,  Somatopleure.  Sp.c, 
Spinal  cord.  Sym,  Sympathetic  ganglion.  V.msn,  Vena  mesonephrica.  W.D,  Wolffian  duct.  W.t, 
Wolffian  tubule.  X  22  diams. 

it  comes  in  contact  with  the  mesentery,  mst.  The  gland  contains  two  kinds  of 
tissue,  one,  the  anlage  of  the  medullary,  the  other  of  the  cortical  portion  of  the 
gland.  The  medullary  tissue  resembles  the  neighboring  mesenchyma  and  occu- 


EMBRYO  OF  17  MM.  237 

pies  only  a  small  territory  about  the  stalk  of  the  organ.  The  cortical  tissue 
contains  cells  with  much  larger  nuclei  and  clearly  developed  protoplasmic  bodies. 
It  occupies  by  far  the  larger  part  of  the  gland.  Comparison  with  figure  122  will 
show  that  the  genital  anlage  at  this  stage  occupies  the  same  topographical  rela- 
tion to  the  Wolffian  body  as  at  earlier  stages.  It  differs  now  from  the  earlier 
condition  chiefly  by  its  growth  in  size  and  by  its  advancement  in  histological 
differentiation.  Below  the  genital  gland  the  intestinal  canal  is  cut  several  times. 
One  portion  of  the  intestine  is  seen  in  the  section  to  be  connected  by  means  of  the 
mesentery,  mst,  with  the  median  dorsal  tissues  of  the  embryo.  The  intestine  is 
formed  by  a  small  tube  of  entoderm  with  a  small  cavity.  The  entoderm  is  a 
rather  thick  cylinder  epithelium.  The  greater  part  in  bulk  of  the  walls  of  the 
intestine  is  constituted  by  mesenchyma.  The  external  surface  is  covered  by 
a  thin  mesothelial  layer.  The  mesenchyma  is  beginning  to  show  the  differentia- 
tion of  the  external  muscular  from  the  internal  mucous  coat.  There  is  at  this 
stage  no  trace  whatever  of  the  development  of  any  folds  or  glands  on  the  inside 
of  the  intestinal  canal. 

Section  through  the  Kidney.- — This  section  being  much  nearer  the  caudal  end 
of  the  embryo,  we  find,  as  throughout  all  the  early  stages,  that  the  differentiation 
of  the  tissues  is  less  advanced  than  nearer  the  head.  We  have  accordingly,  so  to 
speak,  an  earlier  stage  in  the  development  of  the  spinal  cord,  Sp.  c,  of  the  nerves, 
and  of  the  vertebra.  In  the  median  line  is  the  large  aorta,  Ao,  about  which  the 
mesenchyma  is  only  slightly  condensed.  Near  the  aorta  are  the  conspicuous 
anlages  of  the  sympathetic  system,  Sym,  which  appear  at  this  level  in  a  very 
characteristic  hook-shaped  pattern.  At  the  dorsal  end  of  the  hook  the  nerve- 
fibers  are  much  more  numerous  than  in  the  ventral  portion  of  the  anlage.  The 
sympathetic  cells  themselves  are  extremely  conspicuous,  owing  to  the  depth  of 
their  stain.  On  either  side  is  situated  the  anlage  of  the  permanent  kidney.  Each 
anlage  consists  of  an  irregularly  branching  space  bounded  by  a  thick  layer  of 
epithelium,  which  has  somewhat  the  appearance  of  the  intestinal  entoderm  at 
this  stage.  If  the  series  of  sections  be  followed  through  further  toward  the  tail 
of  the  embryo,  the  epithelial  space  will  be  seen  to  contract  to  a  relatively  small 
tube,  the  ureter,  which  opens  into  the  Wolffian  duct  of  the  same  side.  The  ex- 
'panded  portion  of  the  cavity  shown  in  our  figure  corresponds  in  part  to  the  pelvis 
of  the  adult  organ.  Its  irregular  shape  is  due  to  the  fact  that  it  is  forming  a 
series  of  outgrowths,  which  are  to  give  rise  to  the  tubules  of  the  kidney.  Around 
the  ends  of  the  branches  of  the  renal  pelvis  is  a  darker  tissue,  in  which  the  cells 
are  very  much  crowded.  The  nature  of  this  tissue  has  been  much  debated.  Two 
divergent  interpretations  of  it  have  been  offered.  According  to  one  view,  it  is  the 
material  out  of  which  the  glomeruli  and  convoluted  tubules  of  the  kidney  are  to 


238 


STUDY  OF  PIG  EMBRYOS. 


be  differentiated.  By  a  secondary  process  these  tubules  are  supposed  to  become 
united  with  the  branches  from  the  renal  pelvis,  the  branches  forming  only  the 
collecting  tubules  of  the  adult  organ.  According  to  another  view,  the  condensed 
tissue  is  partly  mesenchyma  and  partly  an  outgrowth  from  the  walls  of  the  renal 
epithelium.  This  interpretation  makes  the  collecting  tubules  arise  as  branches 
of  the  pelvis,  and  the  convoluted  tubules  and  glomeruli  as  branches  of  the  collect- 
ing tubules.  The  origin  of  the  renal  anlage  may  easily  be  followed  in  earlier  stages. 
It  is  found  that  from  the  pelvic  end  of  each  Wolffian  duct  there  develops  a  dorsal 


G. 


Sp.c. 


N.' 


Nek. 


N." 


W.  D. 


All. 


FIG.  138. — PIG,  17.0  MM.     TRANSVERSE  SERIES  51,  SECTION  759. 

All,  Allantois.  Ao,  Aorta.  A.um,  Umbilical  artery,  card,  Branch  of  cardinal  vein.  Cce,  Coelom.  G,  Gan- 
glion. .A7,  Kidney.  N'',  N",  Nerves.  Nch,  Notochord.  P.L,  Posterior  limbs.  Rect,  Large  intestine.  Sp.c, 
Spinal  cord.  Sym,  Sympathetic  ganglion.  W.b,  Wolffian  body.  W.D,  Wolffian  duct.  X  *7  diams. 

outgrowth,  which  is  lined  by  epithelium.  This  outgrowth  elongates  in  a  head  ward 
direction.  Its  end  expands;  the  narrow  portion  is  the  ureter,  the  expanded  por- 
tion the  anlage  of  the  pelvis.  The  pelvis  becomes  irregular  in  shape  and  forms 
outgrowths.  Around  it  appears  the  condensed  tissue  just  referred  to.  On  the 
ventral  and  lateral  sides  of  the  kidneys  in  our  section  appear  the  ends  of  the 
Wolffian  bodies,  W .  b.  From  the  ventral  and  inner  edge  of  each  Wolffian  body 
is  a  projecting  lobe  of  tissue  in  which  the  Wolffian  duct,  W.D,  is  lodged.  The 
walls  of  the  Wolffian  duct  are  a  rather  thin,  cuboidal  epithelium,  surrounded  by 


UMBILICAL   CORD   OF  EMBRYO   OF  17  MM.  239 

mesenchyma  in  which  there  is  no  very  clear  evidence  of  specialization.  Between 
.the  Wolffian  bodies  is  suspended  the  large  intestine.  It  has  a  small  canal  formed 
by  entoderm  and  very  thick  mesodermic  walls.  Attached  to  the  ventral  side  of 
the  body-wall  of  the  embryo  is  the  allantois,  All,  the  cavity  of  which  is  quite 
large,  somewhat  irregular  in  shape,  and  lined  by  a  cuboidal  epithelium,  a  portion 
of  the  entoderm.  By  following  through  the  sections  it  can  be  seen  that  the  allan- 
tois and  large  intestine  join  at  the  cloaca.  The  entodermal  allantois  is  sur- 
rounded by  mesenchyma,  which  is  very  much  looser  in  texture  than  that  of  the 
intestine  proper.  On  either  side  of  the  allantois  is  a  projecting  lobe  of  tissue  in 
which  the  umbilical  artery,  A.  um,  is  lodged.  The  two  arteries  pass  upward  to 
the  umbilicus,  then  outward  to  the  placenta.  Downward  they  continue  to  the 
level  of  the  cloaca,  there  pass  to  the  dorsal  side  of  the  embryo,  and  unite  with  the 
end  of  the  median  dorsal  aorta. 

Frontal  Section  of  the  Umbilical  Cord  of  Embryo  of  17  mm. 

Since  the  umbilical  cord  projects  from  the  abdomen,  we  get  in  frontal  series 
of  the  embryo  sections  of  the  umbilical  cord  which  are  more  or  less  nearly  trans- 
verse. Such  sections  are  instructive  (Fig.  139).  In  the  illustration  we  see  that 
the  umbilical  cord  is  formed  chiefly  by-  mesenchyma,  mes.  It  contains  four 
blood-vessels;  two  umbilical  veins,  of  which  the  left,  U.  V.  S,  is  enlarged,  while 
the  right,  U.  V.  D,  is  diminished  in  size.  The  arteries,  Art,  are  almost  symmetri- 
cal in  position  and  alike  in  size.  They  are  much  smaller  than  the  veins,  but  the 
mesenchymal  tissue  about  them  is  somewhat  condensed,  so  that  they  are  provided 
with  an  imperfectly  differentiated  muscular  coat.  The  body-cavity  of  the  em- 
bryo is  prolonged  into  the  cord,  forming  a  central  space,  Cos,  in  which  are  lodged 
the  loops  of  the  intestine  and  the  prolongation  of  the  allantois.  The  intestine  is 
cut  twice,  the  section  on  the  left  passing  through  the  ileum,  and  on  the  right 
through  the  jejunum,  which  is  much  larger  than  the  ileum,  having  both  a  smaller 
entodermal  portion  and  a  thicker  mesenchymal  part.  The  two  segments  of  the 
intestine  are  joined  together,  and  in  the  part  between  them  are  two  blood-vessels, 
one  the  vitelline  artery,  A .  m,  and  the  other  the  vitelline  vein.  The  mesenchyma 
of  the  intestine  and  of  the  bit  of  mesentery  between  them  consists  of  very  crowded 
cells,  so  that  the  tissue  appears  darkly  stained,  and  offers  in  this  respect  a  strik- 
ing contrast  to  the  allantois,  All,  the  mesenchymal  walls  of  which  contain  only 
loosely  scattered  cells.  The.  entoderm  of  the  allantois  is  quite  thin.  The  exter- 
nal surfaces  of  the  intestines  and  of  the  allantois,  and  the  outer  surface  of  the 
coelom,  are  all  lined  by  a  distinct  layer  of  mesothelium.  The  ectoderm,  EC,  is 
thin,  and  consists,  for  the  most  part,  of  a  single  layer  of  cells,  although  the  forma- 
tion of  a  second  outer  layer  seems  to  be  beginning. 


240 


STUDY  OF  PIG  EMBRYOS. 


Pig  Embryo  of  20  mm. 

Sections 'of  this  stage  are  figured.  In  the  practical  laboratory  work  em- 
bryos a  little  larger  or  smaller  may  serve  equally  well  to  illustrate  the  develop- 
mental conditions  of  this  stage. 

Transverse  Section  through  the  Snout. — The  parts  shown  are  the  same  as  in 
figure  146,  to  the  description  of  which  reference  is  made.  The  present  figure  140 


a  V.D. 


u.v.s. 


Art. 


A.vi. 


All. 


EC. 


FIG.  139. — PIG,  17.0  MM.    FRONTAL  SERIES  39,  SECTION  63. 

All,  Allantois.     Art,  Umbilical  artery.    A.vi,  Vitelline  artery.      Cos,  Coelom.     EC,  Ectoderm.     /,;,  lleum.     tries, 
Mesenchyma.     U.  V.D,    Right  umbilical  vein.      U.  V.S,  Left  umbilical  vein.     X  35  diams. 

is  added  to  illustrate  the  development  of  the  palate  shelf,  Pal.  The  palate  shelf  is 
a  large  protuberance  on  the  inner  side  of  the  maxillary  process.  Its  inner  edge 
abuts  against  the  tongue,  Ton,  and  its  upper  edge  underlies  the  maxillo-tui  jinal 
fold,  max.  tb,  and  its  lower  edge  forms  part  of  the  roof  of  the  oral  cavity,  Or.  At 
this  stage  it  consists  of  a  large  mass  of  undifferentiated  mesencnyma,  covered  by 


EMBRYO  OF  20  MM. 


241 


a  layer  of  epithelium.  The  two  palate  shelves  continue  to  grow  toward  one  an- 
other until  they  meet  in  the  median  line  below  the  nasal  septum.  As  they  ap- 
proach one  another  the  tongue  descends.  Ultimately  the  two  palate  shelves 
unite  with  one  another  and  with  the  overlying  nasal  septum.  The  epithelium  of 
the  two  shelves  concresces  and  forms  for  a  time  a  partition,  which  marks  the 
point  of  union  of  the  two  shelves,  both  with  one  another  and  with  the  nasal 
septum.  This  partition  persists  for  a  short  time  only,  for  it  soon  disappears  by 


i  nas.tb. 


Sept. 


tuax.tb. 


Jk.o. 


Ton. 


Pal.   - 


Mk. 


FIG.  140. — PIG,  20  MM.     TRANSVERSE  SERIES  59,  SECTION  522. 

Jk.o,  Jakobson's  organ,  lat,  Lateral  ethmoid  cartilage,  max.tb,  Maxillo-turbinal  fold.  Mk,  Meckel's  cartilage. 
nas.tb,  Naso-turbinal  fold.  Or,  Oral  cavity.  Pal,  Palate  shelf.  Sept,  Cartilage  of  nasal  septum.  Ton, 
Tongue.  X  22  diams. 


resorption.  The  union  of  the  palate  shelves  separates  definitely  the  nasal  and 
oral  cavities  from  one  another.  Their  union  is  gradual,  beginning  in  front 
and  gradually  extending  backward.  It  is  a  not  infrequent  anomaly  that  the 
palate  shelves  fail  to  unite  perfectly.  When  this  occurs,  there  results  the 
condition  known  as  cleft  palate. 

Transverse  Section  through  the  Lower  Part  of  the  Neck  (Fig.  141). — The  spinal 
cord,  Sp.c,  shows  a  very  great  enlargement  of  the  ventral  zones,  which  now  pro- 

16 


242  STUDY  OF  PIG  EMBRYOS. 

ject  downward  so  as  to  enclose  between  them  a  distinct  groove  in  the  median  ven- 
tral line,  which  can  be  identified  as  the  commencing  anterior  fissure  of  the  cord. 
In  this  groove  runs  a  small,  longitudinal  blood-vessel,  the  arteria  sulci,  which  from 
time  to  time  gives  off  small  branches,  which  enter  the  substance  of  the  spinal  cord. 
In  the  ventral  zone  the  ependymal  layer  has  become  quite  thin  and  the  middle  or 
gray  layer  has  acquired  great  thickness,  chiefly  owing  to  the  growth  of  the  neuro- 
blasts,  many  of  which,  especially  toward  the  outside  of  the  cord,  can  now  be 
readily  identified  as  young  nerve-cells.  The  ectoglia  or  outer  neuroglia  layer  has 
increased  in  thickness.  Many  of  the  processes  of  the  neuroglia  cells  can.be 
readily  distinguished,  running,  for  the  most  part,  more  or  less  nearly  perpendicular 
to  the  surface  of  the  cord.  Between  the  neuroglia  fibers  are  numerous  fine  dots 
which  are  the  cut  ends  of  the  nerve-fibers  running  longitudinally.  Although 
about  these  nerve-fibers  there  are  as  yet  no  medullary  sheaths  developed,  it  is, 
nevertheless,  proper  to  speak  now  of  the  ectoglia  as  the  external  white  matter 
of  the  cord.  Immediately  beneath  the  entrance  of  the  dorsal  root  the  external 
outline  of  the  cord  shows  a  concavity  which  disappears  in  later  stages.  The 
dorsal  zones  are  very  much  smaller  than  the  ventral.  The  differentiation  of 
their  three  primary  layers  is  being  completed  by  the  development  of  a  distinct 
middle  layer.  The  ectoglia  of  the  dorsal  zone  resembles  that  of  the  ventral  zone 
in  structure  and  thickness.  The  spinal  ganglia,  G,  have  descended  from  their 
original  position,  so  that  they  now  lie  on  a  level  with  the  lower  edge  of  the  spinal 
cord,  and  the  nerve-root,  by  which  each  ganglion  is  connected  with  the  dorsal  zone 
of  the  cord,  has  correspondingly  elongated.  The  lower  edges  of  the  ganglia  come 
in  contact  with  the  lateral  processes  of  the  vertebra.  Between  the  spinal  cord 
and  the  vertebra  is  an  area  of  loose  mesenchyma  which  may  be  regarded  as  a 
portion  of  the  arachnoid  membrane.  Close  to  the  upper  surface  of  the  vertebra, 
bounded  dorsally  by  the  tissue  just  mentioned,  are  two  symmetrically  placed 
blood-vessels.  The  vertebra,  Vert,  is  distinctly  cartilaginous,  though  not  yet 
fully  differentiated,  and  is  surrounded  by  a  distinct  fibrous  layer,  the  perichbn- 
drium.  In  the  median  line  below  the  vertebra  lie  the  oesophagus,  CE,  and  tra- 
chea, Tra,  both  tubes  lined  by  entoderm.  The  cavity  of  the  oesophagus  is  some- 
what crescent-shaped,  that  of  the  trachea  triangular.  About  the  oesophagus  the 
mesoderm  forms  two  layers,  an  inner  lighter  layer  and  an  outer  muscular  layer, 
the  cells  of  which  are  already  elongated.  The  mesenchyma  about  the  trachea  is 
more  condensed,  especially  on  the  sides  and  below,  and  the  condensed  tissue  is 
in  close  contact  with  the  epithelium.  On  the  dorsal  side  of  the  trachea  close  to 
the  entoderm  is  a  thin  layer  of  transversely  elongated  cells.  The  sympathetic 
nervous  system,  Sym,  appears  symmetrically  placed  near  the  trachea  and  oesoph- 
agus. In  section  the  sympathetic  is  round  and  contains  numerous  nerve-fibers 


EMBRYO  OF  20  MM. 


243 


-5  -X 


244  STUDY  OF  PIG  EMBRYOS. 

and  characteristic  young  sympathetic  nerve-cells,  by  which  it  is  readily  recog- 
nized. Close  to  the  ventral  side  of  the  sympathetic  is  the  section  of  the  large 
jugular  vein,  V.  jug,  a  branch  of  which,  V.  br,  lies  laterad  from  the  main  vessel. 
This  branch  receives  blood-vessels  from  the  facial  region,  and  is  perhaps  the  facial 
vein,  but  its  identity  is  not  certain.  Between  the  main  jugular  and  its  branch 
are  some  lymphatic  spaces,  somewhat  irregular  in  form,  and  lined  by  a  thin  endo- 
thelium  so  that  they  present  a  close  resemblance  to  veins  in  their  structure.  Close 
to  the  medial  wall  of  the  jugular  vein  is  situated  the  large  trunk  of  the  vagus 
nerve,  N.  10.  At  a  little  lower  level  than  the  vagus  nerves  and  in  the  median  line 
lies  the  anlage  of  the  thyroid  gland,  which,  owing  to  its  darker  staining,  is  some- 
what conspicuous.  The  cells  of  the  thyroid  form  ah  irregularly  shaped  branch- 
ing mass.  The  spaces  between  the  branches  are  chiefly  occupied  by  cavities 
lined  by  endothelium  and  which  probably  belong  to  the  lymphatic  system.  The 
arrangement  of  these  cavities  and  the  relation  of  their  endothelium  to  the  cells  of 
the  organ  recall  the  blood  sinusoids  of  the  liver  and  of  the  suprarenal  capsule. 
The  thyroid  cells  are  compactly  arranged  without  distinct  cell-boundaries,  but 
with  protoplasm  which  stains  somewhat  and  with  nuclei  of  rounded  form ,  dis- 
tinct outline,  and  granular  appearance,  the  granules  being  decidedly  more  con- 
spicuous than  the  granules  in  the  nuclei  of  the  neighboring  mesenchymal  cells 
Just  ventral  to  each  jugular  vein  is  a  small  darker  body,  consisting  of  closely 
compacted  cells,  resembling  in  appearance  those  of  the  thyroid.  The  body  has  a 
very  distinct  external  outline  and  is  actively  growing,  for  several  of  its  nuclei  are 
in  mitosis.  The  bodies  in  question  are  the  parathyroid  glands.  The  rest  of  the 
section  is  mainly  occupied  by  mesenchyma  and  numerous  darker  masses,  muse, 
the  anlages  of  the  various  muscles  of  the  neck  and  throat.  On  each  side  is  shown 
a  small  piece  of  the  cartilaginous  scapula,  Scap.  At  the  lower  corner  of  the  sec- 
tion is  an  indication  of  the  anterior  limb,  A .  L,  and  of  its  vein,  Ve". 

Section  through  the  Lungs  (Fig.  142). — The  spinal  cord  shows  very  clearly  in  the 
differentiation  of  the  three  primary  layers  of  the  medullary  wall.  Its  structure  is 
similar  to  that  shown  in  figure  141 ,  and  need  not  be  again  described.  The  vertebra, 
Vert,  is  now  distinctly  young  cartilage.  On  its  ventral  side  its  boundary  is  quite 
distinct,  the  formation  of  the  perichondrium  having  there  begun.  Laterally  it 
merges  into  a  dense  mesenchyma,  by  which  it  is  united  without  demarcation  with 
the  rib,  cost' ,  and  indirectly  with  the  vertebral  arch,  V.  ar,  both  of  which  are 
cartilaginous.  The  cells  of  the  vertebral  cartilage  occupy  rounded  cavities,  each 
of  which  is  marked  by  a  distinct  capsule.  The  matrix  between  the  capsules  is 
homogeneous,  stains  slightly,  and  has  acquired  a  greater  density  than  in  earlier 
stages.  The  cells  themselves  exhibit  traces  of  their  protoplasmic  bodies  and 
have  deeply  stained  nuclei  which  are  quite  irregular  in  shape  and  very  granular. 


EMBRYO  OF  20  MM. 


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246  STUDY  OF  PIG  EMBRYOS. 

Immediately  around  the  notochord  the  spaces  occupied  by  the  cells  are  the 
largest  and  the  capsules  most  distinct,  the  nuclei  most  altered.  Proceeding 
toward  the  periphery  of  the  cartilage,  the  cells  appear  in  successively  earlier  and 
earlier  stages,  until  at  the  very  periphery  we  have  normal  nuclei  and  a  transition 
to  mesenchyma.  The  cells  of  the  notochord  are  beginning  to  degenerate,  and  in 
place  of  the  notochordal  sheath  there  seems  to  be  only  a  space  between  the  noto- 
chordal  cells  and  the  vertebral  cartilage.  Immediately  below  the  vertebra  are  the 
conspicuous  anlages  of  the  sympathetic  system,  Sym.  They  overlie  the  sections 
of  the  posterior  cardinal  veins,  card.  These  are  now  quite  small  vessels,  the  vena 
cava  inferior  having  become  the  main  channel  for  the  return  of  the  blood  from 
the  abdominal  region  to  the  heart.  The  two  cardinal  veins  are  not  quite  sym- 
metrically placed,  that  on  the  left  side  lying  a  little  lower  than  that  on  the  right. 
Between  them  is  situated  the  median  aorta,  Ao,  with  a  relatively  thick  and  well- 
developed  muscular  coat,  the  deeper  staining  of  which  makes  it  conspicuous  even 
with  low  powers.  The  oesophagus,  (E,  and  trachea,  Tra,  are  not  in  the  median 
line,  but  are  both  displaced  toward  the  right  of  the  embryo.  As  compared  with 
earlier  stages,  both  structures  show  an  advance,  first,  by  the  growth  of  the  ento- 
derm;  and,  second,  by  the  differentiation  of  the  surrounding  mesenchyma.  In 
both  oesophagus  and  trachea  the  entoderm  is  a  ring  of  cylinder  epithelium,  the 
tracheal  ring  being  much  larger  than  the  oesophageal.  The  mesenchyma  about 
the  oesophagus  forms  two  distinct  layers,  an  inner  looser  layer  and  an  outer  denser 
muscular  layer.  Around  the  trachea  the  mesoderm  is  much  condensed.  On 
the  dorsal  side  of  the  trachea  the  cells  form  next  to  the  epithelium  a  special  layer 
characterized  by  the  elongated  form  of  the  cells.  Between  the  oesophagus  and 
trachea  are  situated  the  vagus  nerves,  that  of  the  right  side,  N.  10,  occupying  a 
higher  position  than  that  on  the  left,  so  that  the  nerves  are  not  symmetrically 
placed.  The  cardinal  veins,  the  aorta,  the  oesophagus,  the  vagus  nerve,  and  the 
trachea  are  all  imbedded  in  mesenchyma,  which,  together  with  these  structures, 
forms  the  so-called  mediastinum  by  which  the  right  and  left  pulmonary  cavities, 
PI.  d,  PI.  s,  are  separated  from  one  another.  On  its  ventral  side  the  mediastinum 
joins  on  to  the  veins  entering  the  heart.  On  either  side  of  -the  mediastinum 
at  the  level  of  the  trachea  may  be  seen  a  projecting  line.  That  on  the  left  side 
shows  clearly  the  division  of  the  organ  into  a  dorsal  lobe,  Lu.  d,  and  a  ventral 
lobe,  Lu.  -v.  Each  lung  consists  at  this  stage  chiefly  of  mesenchymal  tissue  and 
is  covered  by  a  layer  of  mesothelium  which  forms  the  boundary  of  the  pleural 
ccelom.  Within  the  mesenchyma  appear  several  sections  of  the  branches  of  the 
entodermal  bronchi.  Each  bronchus  is  lined  at  this  stage  by  a  rather  thick  ento- 
dermal  layer  of  cylinder  cells.  The  union  of  the  lung  with  the  mediastinum  con- 
stitutes the  so-called  root  of  the  lung.  In  the  root  of  the  lung  is  seen  the  small 


EMBRYO  OF  20  MM.  247 

pulmonary  artery,  A .  pul.  The  two  arteries  join  a  little  nearer  the  head  and  on 
the  left  side  of  the  embryo  to  form  a  single  trunk,  the  main  pulmonary  artery. 
Originally  the  pulmonary  arteries  arise  symmetrically  as  branches  from  the  fifth 
aortic  arch.  They  soon  unite,  however,  throughout  the  greater  part  of  their 
extent,  forming  a  single  vessel.  The  two  arteries  shown  in  our  figure  represent 
the  two  original  symmetrical  vessels  where  they  are  about  to  enter  the  lungs. 
On  the  ventral  side  of  the  section  various  cardiac  structures  are  shown,  but  so  cut 
that  the  picture  is  not  very  instructive.  It  will  suffice  to  refer  to  the  explanation 
of  the  figure  for  the  identification  of  the  parts. 

Section  through  the  Posterior  Limbs  (Fig.  143). — Although  this  section  is  from 
a  transverse  series,  yet,  owing  to  the  curvature  of  the  body,  it  shows  the  spinal 
cord  cut  very  obliquely.  The  three  layers  of  the  cord,  the  ependymal,  epen,  the 
cinerea  or  neurone  layer,  Cin,  and  the  ectoglia  are  well  marked.  Something  of 
the  dorsal  root,  D.  R,  and  of  the  ganglia,  G,  of  a  lumbar  nerve  are  also  shown  in 
the  section.  The  nerves  have  already  joined  together  to  form  a  very  complex 
lumbar  plexus,  sections  of  portions  of  which  appear  at  various  points.  These  are 
all  indicated  by  the  reference  letter  TV  in  the  figure,  it  being  thought  not  desirable 
to  attempt  an  identification  of  each  component  of  the  plexus.  The  plexus  is  more 
or  less  symmetrically  placed  on  the  right  and  left,  at  about  the  level  of  the  intes- 
tine, Reel.  The  limbs  are  large  projections  extending  downward  and  containing 
in  their  interior  the  cartilaginous  anlages,  cart' ',  cart",  of  the  skeleton  of  the  limb; 
and,  around  these,  darker  masses  of  tissue,  the  developing  muscle-fibers.  At  the 
lower  edge  of  each  limb  is  a  blood-vessel,  V.  p.  the  so-called  border  or  peripheral 
vein,  which  extends  completely  around  the  edge  of  the  developing  hand  and  foot. 
When  the  digits  are  developed,  this  vein  becomes  broken  up,  and  out  of  its  divi- 
sions are  formed  the  digital  vessels.  The  section  also  passes  through  the  penis, 
Pen,  in  the  center  of  which  is  the  urethra,  Ur.  It  shows  here  as  a  narrow  epi- 
thelial band  entirely  without  any  cavity,  except  a  very  small  one  at  its  external 
dorsal  end.  The  band  is  lighter  in  the  center,  owing  to  the  fact  that  the  nuclei 
are  grouped  chiefly  close  to  the  two  surfaces  of  the  band.  At  the  base  of  the  limb 
is  situated  the  irregularly  shaped  section  of  the  iliac  vein,  V.  il.  In  the  median 
line  may  be  noted  the  following  structures.  Immediately  underneath  the  ner- 
vous system  is  the  arteria  sulci,  A.  sul.  The  vertebra,  Vert,  and  notochord, 
Nch,. resemble  corresponding  structures  in  the  section  las.t  described,  except  that 
their  cytomorphosis  is  slightly  less  advanced.  Below  the  vertebra  lie  the  paired 
anlages  of  the  sympathetic  nervous  system,  Sym,  between  which  is  the  small 
median  caudal  artery.  The  intestine,  Rect,  has  its  transverse  diameter  some- 
what increased,  so  that  it  appears  oval  in  the  section.  Around  it  is  beginning  the 
differentiation  of  the  mucosa  muscularis. 


c. 


Nch. 


N. 


N. 


N. 


k  Syrn. 


FIG.  143. — PIG.  20  MM.     TRANSVERSE  SERIES  59,  SECTION  1253. 

arach,  Arachnoid  membrane.  A.sul,  Arteria  sulci.  cart' ,  cart",  Cartilaginous  anlages  of  elements  of  the 
skeleton  of  the  limb.  Cm,  Neurone  layer  of  spinal  cord.  D.R,  Dorsal  root.  EC,  Ectoderm,  epen, 
Ependymal  layer  of  spinal  cord.  G,  Spinal  ganglion,  muse,  One  of  the  muscular  anlages.  N,  N,  N, 
Nerves  of  the  lumbar  plexus.  Nch,  Notochord.  Pen,  Penis.  Rect,  Rectum.  Sk,  Anlage  of  the  dura 
mater.  Sym,  Sympathetic  nerve.  T,  Tail.  Ur,  Urethra.  V.arc,  Vertebral  arch.  Vert,  Vertebra. 
V.il,  Iliac  vein.  F./,"Border  vein  of  the  limb.  X  22  diams. 

248 


EMBRYO  OF  20  MM. 


249 


Section  through  the  Mammary  Anlage. — Figure  144  represents  a  section 
through  the  somatopleure  of  the  embryo  in  the  region  of  a  mammary  gland.  The 
ectoderm,  EC,  covers  the  external  surface  of  the  somatopleure,  as  does  the  meso- 
thelium,  msth,  the  inner  surface,  the  space  between  the  two  covering  layers  being 


Pan. 


Eptr. 


-   Mam. 


%  r 


P1V5,-  '    'iiXWf 


FIG.  144. — PIG,  20.0  MM.     TRANSVERSE  SERIES  59,  SECTION  1043. 

b,  Basement  membrane  of  epidermis,  cost,  Costal  anlage.  Cu,  Cutis.  EC,  Ectoderm.  Eptr,  Epitrichium. 
Mam,  Mammary  anlage.  ines,  Mesenchyma.  msth,  Mesothelium.  Pan,  Tunica  panchoroidea.  per.  m, 
Peritoneal  mesoderm.  ve,  Blood-vessel.  X  25°  diams. 

occupied  by  various  mesodermic  structures.  The  ectoderm  consists  of  two  or 
three  layers  of  cells,  the  external  one  of  which,  Eptr,  the  epitrichium,  is  very  thin. 
To  form  the  mammary  anlage,  Mam,  the  ectoderm  suddenly  thickens  and  pro- 
jects somewhat  upward  and  still  more  downward  into  the  mesoderm.  The  epi- 


250  STUDY  OF  PIG  EMBRYOS. 

trichium  passes  continuously  over  the  thickening,  in  the  production  of  which  it 
takes  no  share.  The  inner  edge  of  the  ectoderm  is  marked  by  a  very  distinct  line 
or  basement  membrane,  b,  against  the  underlying  mesoderm.  The  cells  of  the 
anlage  form  two  groups,  one  a  band  next  to  the  basement  membrane,  in  which 
the  cells  present  a  somewhat  radial  arrangement,  and  the  other  a  central  group 
of  cells,  many  of  which  are  elongated  in  a  direction  somewhat  parallel  to  the  sur- 
face of  the  anlage,  so  that  they  form  curving  lines.  The  elongated  cells  in  later 
stages  gradually  cornify  and  fall  out,  so  that  the  anlage  becomes  hollow,  but  its 
excavation  proceeds  very  slowly,  and  in  man  is  not  usually  completed  until 
after  birth.  Soon  after  the  hollowing  out  of  the  anlage  has  begun,  it  sends 
out  a  series  of  buds  from  its  inner  surface.  These  buds  become  elongated,  some- 
what twisted  cords  of  cells,  and  offer  at  this  stage  resemblance  to  embryonic 
sweat-glands.  The  outgrowths  subsequently  branch  and  develop  central  cavi- 
ties, and  are  ultimately  transformed  into  the  secretory  portion  of  the  gland. 

Figure  144  also  illustrates  some  important  points  in  regard  to  the  differen- 
tiation of  the  somatopleure.  Parallel  to  the  ectoderm,  and  some  distance  from 
it,  is  a  layer,  Pan,  which  is  marked  out  by  numerous  blood-vessels.  This  is  the 
pan-choroid  layer.  There  is  a  slight  but  unmistakable  difference  in  the  mesoderm 
within  and  without  this  layer,  for  in  the  region  between  the  pan-choroid  and  the 
ectoderm  the  cells  are  somewhat  more  crowded.  They  probably  represent  the 
anlage  of  the  cutis,  Cu,  and  of  the  cutis  only.  Within  the  vascular  layer  the 
mesodermic  cells,  mes,  are  less  near  to  one  another,  and  their  processes,  by  which 
they  are  connected,  are  more  slender.  Toward  the  mesothelium  is  a  broad  band 
of  denser  tissue,  cost,  the  rudiment  of  a  rib,  the  inner  boundary  of  which  is  further 
marked  by  several  blood-vessels,  ve.  Between  the  costal  anlage  and  the  meso- 
thelium is  a  layer  of  embryonic  connective  tissue,  the  cells  of  which  are  more 
crowded  toward  the  mesothelium,  so  that  we  may  say  that  there  are  already  im- 
perfectly differentiated  two  layers  of  mesenchyma  within  the  rib.  The  denser 
layer  next  the  mesothelium  is  destined  to  become  still  more  marked  and  to  trans- 
form itself  into  the  connective- tissue  layer  of  the  peritoneum.  With  the  over- 
lying mesothelium  it  develops  into  the  peritoneal  membrane  of  descriptive 
anatomy. 

Sagittal  Section. — Through  the  Right  Lung  and  Kidney  (Fig.  145). — The  lungs 
occupy  a  position  in  the  upper  part  of  the  figure  and  are  easily  recognized  by  the 
conspicuous  entodermal  bronchi,  bro,  which  resemble  in  microscopic  structure 
the  bronchi  of  earlier  stages.  The  branches  are  widely  separated  from  one 
another  by  the  voluminous  mesenchyma  of  the  organ.  The  lung  is  covered  by 
mesothelium,  msth,  and  projects  into  the  pleural  cavity,  Pleu.  c,  which  is  lined 
by  a  continuation  of  the  mesothelium  of  the  lung  itself.  The  pleural  cavity  can 


EMBRYO  OF  20  MM.  251 

be  followed  downward  past  the  Wolffian  body,  W.  b' ,  and  liver,  and  from  there 
past  the  genital  gland,  Gen,  and  so  on  to  the  lowest  part  of  the  abdominal  cavity, 
Ab.  coe.  The  pleural  cavity  at  this  stage  is  entirely  separated  from  the  pericar- 
dial,  but  it  is  still  directly  continuous  with  the  abdominal  cavity.  On  the  ven- 
tral side  (in  the  figure,  to  the  right)  of  the  pleural  cavity  are  the  great  veins,  the 
duct  of  Cuvier,  D.  C,  descending  from  above,  and  the  ductus  venosus,  Du.  V, 
rising  from  below.  The  pleural  cavity  is  separated  from  the  duct  of  Cuvier  by  a 
lamina  of  the  mesoderm,  x,  and  from  the  ductus  venosus  by  a  similar  but  thinner 
lamina,  y.  Both  laminae  are  bounded  on  the  pleural  side  by  the  mesothelium, 
and  on  the  venous  side  by  the  endothelium  of  the  vessel.  The  opening  of  the 
veins  into  the  right  auricle,  Au.  d,  does  not  appear  in  this  section,  though  a  small 
bit  of  the  left  valve,  -v.  s,  which  guards  this  opening  is  shown.  The  Wolffian  body 
is  divided  into  two  parts,  an  upper,  W.  br ,  on  a  level  with  the  liver,  and  a  lower, 
W.  b",  toward  the  pelvic  end  of  the  abdomen.  The  lower  part  is  larger  than  the 
upper.  The  two  parts  are  separated  from  one  another  chiefly  by  the  mesone- 
phric  vein,  V.  msn,  which  is  the  principal  vessel  to  take  the  blood  from  the  Wolff- 
ian body.  It  delivers  the  blood  to  the  lower  end  of  the  vena  cava  inferior.  The 
separation  of  the  two  parts  of  the  Wolffian  body  is,  however,  further  accented  by 
the  position  of  the  genital  gland,  Gen,  and  of  the  kidney,  Ki.  The  structure  of 
the  latter  organ  does  not  differ  much  from  that  of  earlier  stages,  except  that  the 
diameter  of  the  tubules  has  increased.  The  genital  gland  (testis)  is  remarkable 
for  its  large  size.  It  is  covered  by  a  layer  of  mesothelium,  underneath  which  is 
a  rather  broad  layer  of  elongated  mesenchymal  cells,  the  anlage  of  the  tunica 
albuginea.  The  interior  of  the  organ  contains  a  number  of  contorted  epithelioid 
cords  of  cells  in  which  there  are  a  certain  number  of  so-called  primitive  ova,  cells 
which  are  distinguished  by  their  larger  size,  rounded  form,  greater  transparency, 
and  spherical  nuclei.  The  bands  of  cells  are  known  as  the  sexual  cords,  and  they 
are  separated  from  one  another  by  loose  mesenchymal  tissue.  The  cords  fre- 
quently anastomose  with  one  another.  They  are  the  solid  anlages  of  the  semi- 
niferous tubules.  The  question  of  the  origin  of  these  cords  has  been  much  de- 
bated, but  cannot  be  considered  as  yet  settled.  As  to  both  the  origin  and  the 
ultimate  fate  of  the  primitive  ova  in  the  gland  we  have  no  satisfactory  informa- 
tion. The  cords  remain  solid  throughout  embryonic  life,  not  acquiring  a  central 
cavity  until  after  birth.  The  kidney  is  well  defined  and  is  similar  in  structure  to 
the  kidney  in  the  17  mm.  pig.  (page  237),  but  is  somewhat  more  advanced  in  its 
organization,  especially  as  regards  the  formation  of  the  glomeruli  and  convoluted 
tubules.  The  liver  is  a  very  voluminous  organ  permeated  everywhere  by  sin- 
usoidal blood-vessels,  which  offer  the  greatest  possible  variety  in  size.  In  the 
figure  only  the  larger  of  these  blood-vessels  have  been  drawn  in.  A  large  pro- 


D.C.  v.f.     P.cce     A u.i/. 


Pleu.  c. 


MSt/l. 


252 


f'i.'"  Ab.cce. 

FIG.  145. 


FRONTAL  SECTIONS  OF  HEAD,  EMBRYO   OF  20  MM.  253 

portion  of  the  smaller  sinusoids  are  crowded  with  nucleated  red  blood-corpuscles, 
the  nuclei  of  which  are  small  and  deeply  stained ;  hence  each  cluster  of  corpuscles 
stands  out  as  a  darker  spot  in  the  liver,  for  the  liver  cells  themselves  stain  lightly 
and  have  nuclei  which,  though  three  or  four  times  the  size  of  the  nuclei  of  the 
blood-corpuscles,  yet  appear  relatively  pale  in  the  stained  specimen.  The  blood- 
corpuscles  which  form  the  clusters  in  the  liver  differ  somewhat  from  those  in  the 
active  circulation,  for  they  are  smaller  and  show  less  of  the  characteristic  hemo- 
globin color.  It  is  believed  that  the  liver  at  this  stage  furnishes  sites  for  the 
multiplication  of  the  blood-corpuscles,  and  the  clusters,  which  are  so  conspicuous 
in  the  organ,  correspond  not  to  blood-corpuscles  in  active  circulation,  but  rather 
to  corpuscles  which  have  found  a  lodging-place  in  the  liver  and  are  there  prolifer- 
ating. Our  knowledge  of  the  blood-forming  function  of  the  embryonic  liver  is 
imperfect.  Above  the  liver  is  the  septum  trans versum  or  diaphragm,  Diaph' ', 
Diaph" ',  which  is  formed  chiefly  by  mesenchyma.  On  the  lower  side  of  the  liver 
is  another  broad  accumulation  of  mesenchyma,  mes,  in  which  is  lodged  the  gall- 
bladder, a  small  section,  G.  bl,  of  the  entodermal  lining  of  which  is  included.  The 
intestine,  In',  In",  In'" ,  is  cut  several  times,  because  at  this  stage  the  intestinal 
canal  forms  several  coils  in  the  abdominal  cavity  below  the  liver  and  on  the  ven- 
tral side  of  the  Wolffian  bodies.  Below  the  intestines  appear  the  curious  meso- 
thelial  villi,  All.  m,  of  the  allantois  (compare  page  222).  At  this  stage  the  villi 
are  little  more  than  large  vesicles  of  mesothelium,  which  contain  in  their  interior 
some  coagulum  and  a  very  few  mesenchymal  cells,  associated  with  which  are  a 
few  fibers — whether  true  connective-tissue  fibers,  or  not,  is  undetermined.  The 
mesothelium  of  the  villi  is  a  very  thin  layer  of  flattened  cells. 

Frontal  Sections  of  the  Head. — The  three  sections  to  be  described  are  from  a 
special  series  of  the  head.  The  plane  of  the  series  was  made  as  nearly  as  possible 
transverse  and  at  right  angles  to  the  plane  of  the  roof  of  the  mouth.  They  illus- 
trate some  important  points  in  regard  to  the  development  of  the  facial  region  and 
of  the  fore-brain.  In  all  of  the  sections  the  differentiation  of  the  mesoderm 
around  the  brain  is  clearly  demonstrated.  The  pia  mater  is  very  distinct.  In 
those  parts  of  the  sections  where  the  brain- wall  is  cut  obliquely,  it  can  be  dis- 


FIG.   145. — PIG,  20.0  MM.     SAGITTAL  SERIES  60,  SECTION  213. 

Ab.cce,  Abdominal  coelom.  All.vl,  Mesothelial  villi  of  the  allantois.  Au.d,  Right  auricle,  bra,  Entodermal 
bronchus.  Cce',C(e",  Coelom.  D.C,  Duct  of  Cuvier.  Diaph' ',  Diaph",  Diaphragm.  Du.v,  Ductus  ve- 
nosus.  G.bl,  Gall-bladder.  Gen,  Genital  gland.  In',  In",  In'",  Intestine.  Ki,  Kidney.  Li,  Liver. 
mes,  Mesenchyma.  msth,  Pleural  mesothelium.  P.cce,  Pleural  coelom.  Pleu.c,  Pleural  cavity.  Ve,  Vein 
of  liver.  V.  msn,  Vena  mesonephrica.  v.s,  Valvula  sinistra.  W.b' ,  W.b" ',  Wolffian  body,  x,  Partition, 
separating  the  pleural  cavity  from  the  duct  of  Cuvier.  Y,  Partition  separating  the  pleural  cavity  from  the 
ductus  venosus.  X  22  diams. 


254 


STUDY  OF  PIG  EMBRYOS. 


tinguished  only  by  a  somewhat  careful  observation,  as  the  tissues  of  the  pia  mater 
and  of  the  brain  overlap.  All  about  the  brain  is  the  broad  zone  of  the  arachnoid 
(Figs.  147  and  148,  arach),  easily  distinguishable  even  with  a  low  power  by  its 
light  coloration.  It  consists  of  widely  separated  cells  connected  together  by 
very  distinct  processes,  and  is  permeated  by  a  number  of  small  blood-vessels 
running  in  various  directions  through  the  layer.  Its  external  boundary  is  now 
very  distinct,  being  marked  by  a  layer  of  somewhat  crowded,  elongated  cells 
which  merge  on  the  side  toward  the  ectoderm  into  the  general  surrounding  mesen- 


Max. 


nas.tb. 


max.tb. 


MX.  sup. 


Mdb. 


FIG.  146. — PIG,  20.0  MM.     FRONTAL  SECTION  OF  HEAD.     SERIES  40,  SECTION  68. 

H,  Cerebral  hemisphere.  Jk.o,  Jakobson's  organ.  Max,  Maxillary  process.  max.tb,  Maxilloturbinal  fold. 
Mdb,  Mandible.  MX. sup,  Superior  maxillary  nerve.  nas.tb,  Naso-turbinal  fold.  Sept,  nasal  septum. 
Sk,  Mesenchymal  anlage  of  the  dura  mater  and  skull.  X  "8  diams. 

chyma.     Out  of  this  denser  layer  (Figs.  146  and  147,  Sk}  arise  both  dura  mater 
and  the  membrane  bones  of  the  skull. 

Section  through  the  Anterior  Part  of  the  Snout  (Fig.  146). — On  the  dorsal  side 
appear  the  two  hemispheres,  H,  cut  separately  and  each  showing  the  cavity  of 
its  lateral  ventricle.  On  the  ventral  side  the  mandible,  Mdb,  is  cut  separately 
and  is  separated  by  the  oral  fissure  from  the  rest  of  the  section.  The  maxillary 
processes,  Max,  are  large,  and  each  is  furnished  with  an  inward  prolongation 


FRONTAL  SECTIONS  OF  HEAD,  EMBRYO  OF  20  MM.  255 

extending  toward  the  median  line.  From  the  oral  fissure  there  extend  upward 
•  two  irregular  cavities,  the  nasal  chambers.  The  two  cavities  are  separated  from 
one  another  by  a  broad  mass  of  tissue,  the  nasal  septum,  the  ventral  edge  of  which 
at  this  stage  forms  a  portion  of  the  roof  of  the  mouth-cavity.  In  the  center  of  the 
nasal  septum  is  a  broad  band,  Sept,  of  denser  mesenchymal  tissue,  the  anlage  of 
the  cartilaginous  septum  of  the  nose.  On  either  side  of  the  nasal  septum  are  the 
two  irregularly  shaped  nasal  cavities,  which  open  into  the  mouth  between  the  ven- 
tral edge  of  the  nasal  septum  and  the  inner  edge  of  the  maxillary  process.  The 
medial  side  of  each  nasal  cavity  is  comparatively  regular,  but  the  external  side 
shows  two  prominences,  each  of  which  is  formed  by  a  mass  of  mesenchymal  tissue 
covered  by  epithelium.  The  upper  of  these  projections,  nas.  tb,  is  the  anlage  of 
the  naso-turbinal  fold,  and  the  lower,  max.  tb,  the  anlage  of  the  maxillo-turbinal 
fold.  In  the  nasal  septum  itself  are  two  oval  rings  of  epithelium,  sections  of 
Jakobson's  organs.  This  organ  is  an  evagination  of  the  epithelial  lining  of  the 
nasal  cavity  which  opens  anteriorly  and  extends  backward  some  distance  in  the 
nasal  septum.  In  the  maxillary  process  may  be  observed  the  maxillary  nerve. 
The  number  of  cells  in  the  nerve  has  increased,  and  consequently  the  division  of 
the  nerve-fibers  into  distinct  bundles  has  become  more  marked  as  compared  with 
the  pig  embryo  of  12  mm. 

Section  through  the  Middle  of  the  Snout  (Fig.  147). — The  relations  are  very 
similar  to  those  described  in  the  previous  section,  so  that  it  will  suffice  to  note  the 
three  most  important  differences :  First,  the  absence  of  Jakobson's  organ ;  second, 
the  appearance  of  the  tongue,  Ton,  and  third,  of  the  olfactory  nerve,  N.  olf.  The 
tongue  is  a  protuberance  attached  to  the  lower  jaw,  Mdb.  Its  connection  with 
the  jaw  is  rather  narrow  and  corresponds  to  the  frenum.  The  tongue  extends 
upward  between  the  maxillary  processes  until  it  is  almost  or  quite  in  contact  with 
the  lower  edge  of  the  nasal  septum.  It  is  formed  by  a  somewhat  dense  mass  of 
tissue  in  which  there  is  no  very  evident  histological  differentiation,  and  is  covered 
by  a  layer  of  epithelium  of  moderate  thickness  and  which  is  probably  entirely 
derived  from  the  entoderm,  for  the  tongue  first  appears  as  a  small  median  pro- 
tuberance on  the  ventral  floor  of  the  pharynx,  between  the  first  gill  pouches. 
The  olfactory  nerve,  N.  olf,  can  be  seen  joining  the  lower  part  of  the  inner  side  of 
the  brain-wall  and  extending  down  toward  the  nasal  cavity  and  branching. 
Under  the  part  of  the  nerve  near  the  brain- wall  numerous  cells  are  mingled  with 
the  fibers,  and  by  their  crowding  render  the  nerve  conspicuous  in  stained  sections. 
The  fibers  of  the  olfactory  nerve  differ  from  all  other  nerve-fibers  known  in  ver- 
tebrates. They  arise  as  prolongations  of  certain  of  the  epithelial  cells  of  the 
olfactory  region  of  the  nose  and  grow  from  these  cells  into  the  brain,  where  they 
have  their  termination  in  the  glomeruli  of  the  olfactory  bulb.  All  other  nerve- 


256 


STUDY  OF  PIG  EMBRYOS. 


fibers  arise  from  nerve-cells  either  of  the  ventral  nervous  system  or  of  the  gan- 
glia. Morphologically,  therefore,  the  olfactory  nerve  takes  a  unique  place,  and 
is  not  directly  comparable  with  any  other  nerve  of  the  brain.  The  cells  which 
accumulate  in  the  course  of  the  olfactory  nerve  do  not,  so  far  as  known,  have  any 
direct  share  in  the  production  of  the  nerve-fibers.  Nor  do  they  result  in  the 
formation  of  the  medullary  sheaths,  as  they  do  in  other  nerves,  the  olfactory 
nerve-fibers  remaining,  as  it  is  termed,  naked  throughout  life. 

arach.       Sk. 


Mdb. 


FIG.  147. — PIG,  20.0  MM.     FRONTAL  SECTION  OF  HEAD.     SERIES  40,  SECTION  84. 

arach,  Arachnoid  membrane.  H,  Cerebral  hemispheres.  Max.tb,  Maxilloturbinal  fold.  Mdb,  Mandible.  MX. 
sup,  Superior  maxillary  nerve.  Nas.tb,  Naso-turbinal  fold.  N.olf,  Olfactory  nerve.  Sept,  Cartilaginous 
septum  of  the  nose.  Sk,  Mesenchymal  anlage  of  the  dura  mater  and  skull.  Ton,  Tong'ue.  X  *8  diams. 

Section  through  the  Fore-brain  and  Eyes  (Fig.  148). — The  section  passes 
behind  the  nasal  cavities,  no  part  of  which  is  shown.  The  maxillary  and  mandi- 
bular  processes  are  united  and  the  pharynx,  Ph,  appears  as  a  closed  cavity.  On 
the  dorsal  side  of  the  section  the  fore-brain  stands  out  conspicuously,  both  from  its 
dark  staining  and  from  being  surrounded  by  the  lightly  stained  broad  zone  of  the 
arachnoid,  arach.  The  cavity  of  the  fore-brain  has  two  lateral  expansions,  L.  V, 


FRONTAL  SECTIONS  OF  THE  HEAD,  EMBRYO  OF  20  MM. 


257 


the  lateral  ventricles,  which  extend  outward  and  upward.  The  walls,  H,  of  the 
lateral  ventricles  are  much  thinner  than  the  walls  of  the  lower  part  of  the  fore- 
brain  and  are  the  anlages  of  the  cerebral  hemispheres.  In  the  median  plane  the 
hemispheres  include  between  themselves  a  partition,  Fx,  of  mesodermic  tissue 


arach.    Sk. 


FJC. 


L.V. 


Fix. 


C.str. 


m.rec.sup. 
m.retr.b. 


hy.gl. 


art. 


FIG.  148. — PIG,  20  MM.     FRONTAL  SECTION  OF  HEAD.     SERIES  40,  SECTION  123. 

arach,  Arachnoid  zone,  art,  Lingual  arteries.  C.str,  Corpus  striatum.  ec.gl,  Ectoglia.  Fx,  Falx  cerebri.  H, 
Cerebral  hemisphere,  hy.gl,  Hyoglossal  muscle.  L,  Lens.  L.  V,  Lateral  ventricle.  Mk,  Meckel's 
cartilage,  m.rec.sup,  Musculus  rectus  superior,  m.retr.b,  Musculus  retractor  bulbi.  m.r.lat,  Musculus 
rectus  lateralis  (cf.  text).  Mx.i,  Inferior  maxillary  nerve.  Ph,  Pharynx.  Fix,  Plexus  choroideus  lateralis. 
Ret,  Retina.  Sk,  Anlage  of  membranous  skull.  Ton,  Tongue,  x,  Unidentified  structure.  X  1 8  diams. 

which  can  at  once  be  identified  as  the  falx.     From  the  base  of  the  falx  there  ex- 
tends on  each  side  a  fold,  Plx,  which  projects  into  the  cavity  of  the  lateral  ventri- 
cle.    This  fold  contains  in  its  interior  a  prolongation  of  the  mesodermic  tissue  of 
17 


258  STUDY  OF  PIG  EMBRYOS. 

the  falx,  and  it  is  covered  by  a  continuation  of  the  wall  of  the  hemispheres.  The 
covering  layer  of  the  fold  is  much  thinner  than  any  other  portion  of  the  brain- 
wall  shown  in  the  section,  and  shows  no  differentiation  into  layers.  It  retains 
throughout  life  an  epithelial  character  and  is  already  to  be  termed  ependyma. 
The  ependyma  of  the  two  folds  is  connected  across  the  median  line,  and  it  forms 
the  median  dorsal  boundary  of  the  cavity  of  the  fore-brain.  The  two  folds  are  the 
anlages  of  the  lateral  choroid  plexus.  They  are  destined  to  grow  much  in  size 
and  in  complexity  of  form,  but  they  always  remain  morphologically  what  they 
now  are,  vascularized  mesenchyma  covered  by  ependyma.  The  choroid  plexus 
protrudes  into  the  cavity  of  the  brain  in  the  same  way  in  which  the  viscera  may 
be  said  to  protrude  into  the  abdominal  cavity.  The  cavity  of  the  brain  is 
bounded  by  the  brain-wall  or  ependyma,  just  as  the  abdominal  cavity  is  bounded 
by  the  peritoneum.  The  vascular  tissue  of  the  choroid  plexus  is  outside  of  the 
cavity  of  the  brain,  in  the  same  way  that  the  tissue  of  the  kidney  is  outside  the 
cavity  of  the  abdomen.  Throughout  life  the  choroid  plexus  springs,  as  it  does 
from  the  start,  from  the  medial  wall  of  the  hemispheres,  and  it  is  only  at  that 
point  that  it  can  receive  its  blood-supply.  The  lateral  walls  of  the  hemispheres, 
H,  gradually  thicken  as  they  continue  ventralwards,  and  on  the  ventral  side  of 
the  brain  form  in  part  the  lateral  boundary  of  the  medial  portion  of  the  brain- 
cavity,  as  an  especial  thickening  of  the  brain-wall  which  projects  far  into  the 
cavity.  The  thickening,  C.  sir,  is  the  corpus  striatum.  Between  the  summit  of 
the  corpus  striatum  and  the  choroid  plexus  is  an  open  passage  through  which 
we  may  pass  from  the  median  portion  of  the  brain-cavity  into  the  lateral  ventri- 
cle, L.  V.  The  passage  is  the  foramen  of  Munro,  which  we  learn  from  this  section 
is  bounded  above  by  the  choroid  plexus,  and  below  by  the  corpus  striatum.  On 
the  dorsal  and  middle  sides  of  the  hemispheres,  the  ectoglia,  ec.  gl,  is  already 
clearly  differentiated.  There  is,  however,  at  this  stage,  no  clear  indication  of  the 
cortex  cerebri,  although  in  the  slightly  older  stages  it  will  begin  to  develop  by 
the  accumulation  of  neuroblasts  immediately  beneath  the  ectoglia.  The  noto- 
chord  does  not  appear  between  the  brain  and 'the  pharynx,  the  section  being  too 
far  forward.  The  notochord  stops  near  the  hypophysis.  The  eyes  are  not  cut 
quite  symmetrically.  They  show  the  lens,  L,  and  retina,  Ret,  clearly,  and  the 
left  eye  of  the  embryo  shows  also  the  entrance  of  the  optic  nerve.  On  the  right 
side  of  the  embryo,  near  the  eye,  are  three  areas  which  are  somewhat  more 
darkly  stained  than  the  surrounding  mesenchyma.  These  are  the  anlages  of  the 
muscles  of  the  eye.  They  have  not  yet  been  studied  sufficiently  to  make  their 
identification  certain,  but  it  seems  probable  that  the  uppermost  of  these  anlages, 
m.  rec.  sup,  is  the  rectus  superior,  that  the  middle  one,  m.  retr.  b,  is  the  retractor 
bulbi,  and  that  the  lowest  one,  m.  r.  lat,  is  the  rectus  lateralis.  Until  a  recon- 


FRONTAL  SECTIONS  OF  TffE  HEAD,  EMBRYO  OF  24  MM.         259 

struction  is  made  these  identifications  can  be  recorded  as  tentative  only.  The 
pharynx,  Ph,  appears  as  a  yolk-shaped  slit  lined  throughout  by  entoderm.  From 
its  median  ventral  floor  rises  the  great  mass  of  the  tongue,  Ton,  over  which  the 
dorsal  roof  of  the  pharynx  forms  a  closely  fitting  arch.  A  portion  of  the  epi- 
thelium of  the  tongue  is  loosened  from  the  underlying  tissue,  probably  owing  to 
defective  preservation.  Upon  the  lower  side  of  the  tongue  extend  downward  the 
anlages  of  the  hyoglossal  muscles,  hy.  gl,  between  which  are  situated  the  lingual 
arteries,  art.  On  either  side,  in  the  part  of  the  section  corresponding  to  the  man- 
dible, appears  Meckel's  cartilage,  Mk,  a  somewhat  conspicuous  and  easily  identi- 
fied structure,  owing  to  its  dark  staining.  Meckel's  cartilage  is  the  primitive 
skeletal  element  of  the  mandibular  arch,  and  is  homologous  with  the  cartilagin- 
ous jaw  of  the  lower  fishes.  It  is,  for  the  greater  part,  a  purely  embryonic  struc- 
ture, the  mandible  of  the  adult  being  a  secondary  bone.  By  referring  to  figure 
99  (pig  10  mm.),  it  can  be  seen  that  the  mandibular  arch  extends  upward  to- 
ward the  otocyst  and  forms  the  boundary  of  the  first  gill  cleft,  the  outer  division 
of  which  becomes  the  meatus  auditorius  externus.  In  other  words,  the  upper 
portion  of  the  mandibular  arch  is  in  close  proximity  to  the  otocyst  and  the  anlage 
of  the  tympanum  or  middle  ear.  Meckel's  cartilage  is  a  rod-like  structure  ex- 
tending the  entire  length  of  the  arch.  Its  upper  end  is,  therefore,  close  to  the 
future  tympanum.  While  the  greater  part  of  Meckel's  cartilage  disappears 
during  later  development,  the  upper  end  persists  and  takes  a  direct  share  in  the 
formation  of  the  malleus.  A  little  outside  of  Meckel's  cartilage  in  our  section 
is  the  inferior  maxillary  nerve,  MX.  i,  and  still  further  lateralwards  is  a  small, 
darkly  stained  body,  x,  which  has  not  yet  been  identified  with  certainty. 

Pig  Embryo  of  24  mm. 

Section  through  the  Eye. — In  the  pig  of  24  mm.  the  anlages  of  all  the  parts  of 
the  adult  eye  may  be  said  to  be  present,  with  the  exception  of  the  pigment  layer 
of  the  iris,  which  arises  somewhat  later  by  a  forward  growth  of  the  retina  and 
pigment  layer.  The  origin  of  the  retina  and  lens  is  illustrated  by  the  chicken 
embryo  (Figs.  155,  156),  and  in  a  more  advanced  stage  by  the  pig  of  12  mm.  (Fig. 
1 1 6) .  There  is  added  here  figure  1 49 ,  from  a  section  of  the  eye  of  a  rabbit  embryo 
of  thirteen  days,  in  order  to  facilitate  the  comparison  between  the  12  mm.  stage 
and  the  24  mm.  stage  of  the  pig.  In  figure  149  the  ectoderm,  EC,  forms  an  arch 
over  the  eye  and  indicates  the  commencing  formation  of  the  cornea,  the  layer  of 
ectoderm  being  destined  to  become  the  external  epithelium  of  the  cornea.  Be- 
tween the  lens  and  the  retina  there  has  been  an  ingrowth  of  tissue  accompanied 
by  blood-vessels,  which  form  a  more  or  less  distinct  covering  over  the  surface  of 
the  lens  and  constitute  the  so-called  tunica  vasculosa,  tu.  i>.  The  space  between 


260 


STUDY  OF  PIG  EMBRYOS. 


the  retina  and  lens  will  increase  during  the  following  stages  and  will  become  oc- 
cupied by  a  verj  clear  tissue  containing  a  minimal  .number  of  cells.  This  clear 
tissue  is  the  commencement  of  the  vitreous  humor.  Between  the  lens  and  the 
overlying  ectoderm  the  mesenchyma  has  begun  to  penetrate.  This  mesenchyma 
will  ultimately  furnish  the  connective  tissue  of  the  cornea  and  of  the  iris. 
About  the  eyeball  as  yet  there  is  no  distinct  condensation  of  tissue  such  as  will  ap- 
pear later  to  develop  the  anlages  of  the  choroid  and  scleral  coats  of  the  eyeball. 


EC,  Epidermis. 


FIG.  149. — RABBIT  EMBRYO  OF  THIRTEEN  DAYS  ;  SECTION  OF  THE  EYE. 
Z,  Lens,     tries,  Mesenchyma.     N,  Anlage  of  optic  nerve.     P,  Pigment  layer. 
'  tu.v,  Tunica  vasculosa  lentis. 


,  Retina. 


In  the  pig  of  24  mm.  (Fig.  150)  we  encounter  a  marked  advance  hr  the  differ- 
entiation of  all  parts  of  the  eye.  Above  and  below  the  eye  the  anlages  of  the  eye- 
lids, L.sup,  L.inf,  have  appeared.  The  anlage  is  at  this  stage  merely  a  projecting 
fold  of  the  ectoderm  filled  with  mesenchyma  and  extending  a  short  distance  over 
the  projecting  cornea.  The  folds  will  continue  to  grow  until  the  eyelids  meet  in 
the  middle  of  the  eye,  covering  it  completely.  The  ectoderm  of  the  two  lids  where 
they  meet  unites.  The  union  of  the  two  lids  occurs  in  all  mammals,  and  in  some 


EMBRYO  OF  24  MM. 


261 


cases  they  do  not  separate  again  until  after  birth,  in  which  case  the  animals  are 
said  to  be  "  born  blind."     The  ectoderm,  EC,  of  the  cornea  describes  a  wide,  high 


L.sup. 


N.I.         N.op. 


L.inf. 


~  an.ch. 


Schl. 


FIG.  150. — PIG,  24.0  MM.     TRANSVERSE  SERIES  62,  SECTION  428. 

an.ch,  Anterior  chamber  of  eye.  corn,  Corneal  mesoderm.  EC,  Ectoderm  (epidermis).  Ep,  Inner  epithelium 
of  cornea.  Ir,  Mesodermal  anlage  of  iris.  L' ,  Outer  layefr  of  lens.  L" ',  Inner  layer  of  lens.  L.inf, 
Inferior  eyelid.  L.sup,  Superior  eyelid.  N.$,  Oculomotor  nerve.  N.op,  Optic  nerve.  Pig,  Pigment 
layer.  Ret,  Retina.  Schl,  Canal  of  Schlemm.  tu.v,  Tunica  vasculosa  lentis.  Vit,  Vitreous  humor. 
X  50  diams. 


arch,  underneath  which  is  a  broad  band  of  embryonic  connective  tissue,  corn, 
which  forms  the  main  thickness  of  the  cornea.     Between  the  connective  tissue 


262  STUDY  OF  PIG  EMBRYOS. 

of  the  cornea  and  the  anterior  surface  of  the  lens  is  a  clear  space,  an.  ch,  which  we 
can  identify  as  the  anterior  chamber  of  the  eye,  which  in  the  adult  is  filled  only 
with  the  aqueous  humor.  On  the  corneal  side  the  anterior  chamber  is  bounded 
by  a  distinct  layer  of  cells,  Ep,  the  internal  epithelium  of  the  cornea.  This 
layer  is,  however,  formed  from  the  mesenchyma,  the  cells  of  which  develop 
into  the  internal  epithelioid  covering  of  the  cornea.  At  the  upper  and  lower 
edge  of  the  cornea  there  is  a  separate  forward  growth,  Ir,  of  the  connective 
tissue  between  the  cornea  and  the  lens.  It  is  the  anlage  of  the  connective- 
tissue  layer  of  the  iris.  In  later  stages  it  will  grow  still  further  over  the 
lens  from  all  sides,  leaving  a  central  opening,  the  pupil,  and  it  will  acquire 
a  special  pigmented  layer  on  its  side  nearest  the  lens.  At  the  base  of  the 
iris  anlage  is  a  small  blood-vessel,  Schl,  which  is  commonly  designated  in 
the  adult  as  the  canal  of  Schlemm.  The  retina  has  increased  in  thick- 
ness and  is  closely  covered  by  a  pigment  layer,  Pig.  The  separation  which 
appears  on  the  inner  side  of  the  eyeball  between  the  retina '  and  pigment  layer 
in  figure  150  is  probably  artificial,  the  result  of  shrinkage  during  the  preserva- 
tion of  the  specimen.  At  its  outer  edge  the  retina,  Ret,  suddenly  thins  out 
and  passes  over  into  the  external  pigment  layer,  which  is  heavily  loaded 
with  dark,  uniform,  pigment  granules,  especially  crowded  together  on  the  side 
of  the  layer  nearest  the  retina.  In  later  stages  the  pigment  layer  grows  for- 
ward on  the  inner  side  of  the  iris,  making  a  fold,  so  that  the  iris  is  covered  on  the 
inside  by  a  double  layer  of  pigmented  epithelium,  the  wvea.  The  retina  resembles 
closely  in  structure  the  brain- wall  in  an  early  stage,  for  it  has  on  its  outer  surface 
a  thin  layer  corresponding  to  the  ectoglia,  and  within  a  broad,  nucleated  zone. 
The  mitotic  figures  are  found  only  next  to  the  surface  of  the  retina  nearest  the 
pigment  layer.  Since  the  space  between  the  pigment  layer  and  the  retina  corre- 
sponds to  the  cavity  of  the  brain,  it  is  evident  that  the  position  of  the  mitotic 
figures  is  homologous  with  their  position  in  the  medullary  wall  elsewhere.  The 
section  of  the  lens  clearly  reveals  its  vesicular  structure.  The  external  wall  of 
the  lens  vesicle,  L",  is  a  comparatively  thin  epithelial  layer  which  stains  quite 
readily  and  therefore  stands  out  clearly  in  the  section.  Toward  the  edges  of  the 
lens  the  outer  layer  slightly  thickens  and  then  passes  over  quite  abruptly  into 
the  inner  layer  of  the  vesicle,  L',  which  is  very  thick  and  constitutes  by  far  the 
greater  part  of  the  bulk  of  the  organ  and  gives  to  the  lens  its  characteristic  shape. 
The  outer  and  inner  walls  of  the  lens  are  in  close  contact  so  that  there  is  no  actual 
cavity  present.  The  epithelial  cells  of  the  inner  wall  have  elongated  enormously, 
so  much  that  they  might  perhaps  already  be  termed  "  fibers."  Each  cell  is  sup- 
posed to  extend  through  the  entire  thickness  of  the  inner  wall.  The  nuclei  are 
placed  somewhat  irregularly  in  the  middle  portion  of  the  long  cells  so  that  they 


EMBRYO  OF  24  MM.  263 

constitute  a  more  or  less  distinct  band  in  the  section.  Toward  the  edge  of  the 
lens  the  nuclear  band  becomes  more  distinct,  and  where  the  inner  wall  merges 
into  the  outer,  the  band  becomes  narrow  and  the  nuclei  are  much  crowded  to- 
gether. The  nuclei  of  the  lens  fibers  are  oval,  being  slightly  elongated  in  the  same 
direction  as  the  fibers,  and  each  nucleus  contains  usually  a  distinct  nucleolus. 
Between  the  lens  and  the  retina  is  the  vitreous  humor,  Vit,  which  has  become 
quite  voluminous.  It  contains  a  few  mesenchymal  cells  and  a  few  small  blood- 
vessels, and  when  examined  with  a  high  power  it  is  seen  to  be  permeated  by  a 
fine  network  which  is  probably  to  be  interpreted  as  a  modification  of  the  proto- 
plasmic threads  of  the  mesenchyma.  There  are  also  a  very  few  cells  of  rounded 
form  and  distinct  outline,  with  a  single  small  granular  nucleus,  which  are  probably 
leucocytes.  Against  the  surface  of  the  lens  there  is  a  delicate  homogeneous 
/nyaloid  membrane,  which  can  usually  be  better  seen  where  by  shrinkage  it  has 
been  loosened  from  the  surface  of  the  lens,  as  is  apt  to  occur.  Against  the  hya- 
loid membrane  are  a  number  of  small  blood-vessels,  more  numerous  than  those 
elsewhere  in  the  vitreous  humor,  and  forming  a  fairly  distinct  vascular  mem- 
brane around  the  lens.  The  membrane,  tu.  v,  is  called  the  tunica  vasculosa 
lentis.  The  blood-vessels  of  the  vitreous  humor  are  chiefly,  possibly  at  this  stage 
exclusively,  branches  of  the  central  artery  of  the  retina.  The  artery  enters  the 
eye  through  the  optic  nerve,  and  sends  branches  throughout  the  vitreous  humor. 
The  space  originally  occupied  in  the  humor  by  the  stem  of  the  central  artery 
persists,  and  is  called  the  hyaloid  canal.  The  muscles  of  the  eye  are  already 
differentiated,  but  their  relations  cannot  be  properly  understood  without  a  recon- 
struction. 

Median  Sagittal  Section  (Fig.  151 ) . — The  section  figured  is  very  nearly  median 
for  the  region  of  the  head,  but  in  the  body  it  passes  to  the  left  of  the  median  plane. 
The  area  occupied  in  the  section  by  the  neck  and  head  of  the  embryo  is  almost  as 
great  as  that  occupied  by  the  rest  of  the  body.  The  great  size  of  the  head  at  this 
stage  is  characteristic.  Attention  is  especially  directed  to  the  sharp  angle  which 
the  medulla  oblongata,  Md.  ob,  makes  with  the  spinal  cord,  Sp.  c,  and  to  the  very 
great  bend  formed  by  the  floor  of  the  mid-brain,  Ar.  hab,  in  consequence  of  which 
the  floor  of  the  hind-brain  above  the  medulla  oblongata  and  the  floor  of  the  fore- 
brain  are  brought  quite  close  together  and  run  in  almost  parallel  directions.  The 
cavity  of  the  brain  is  very  large.  Its  walls  in  the  median  plane  are,  for  the  most 
part,  thin.  From  the  roof  of  the  diencephalon,  Dien,  there  runs  off  a  small  eva- 
gination,  Ephys,  a  shallow  pocket  or  diverticulum  of  the  medullary  wall.  It  is 
the  anlage  of  the  epiphysis  or  pineal  organ  of  the  adult.  It  is  an  important  land- 
mark in  the  topography  of  the  brain,  for  its  position  is  always  at  the  extreme 
limit  of  the  fore-brain.  In  the  wall  of  the  mid-brain,  behind  the  epiphysis,  for 


Fix.  IV. 


Cbl 


M.b. 


Ar.hab. 


Epen. 


Ven.IV. 


Rphys. 


Dien. 


Plx. 


Cce. 


IV.  b. 


264 


EMBRYO  OF  24  MM.  265 

some  distance  the  ectoglia  shows  considerable  thickening  and  contains  a  very 
large  number  of  nerve-fibers  running  transversely.  They  constitute  the  poste- 
rior commissure,  which  morphologically  belongs  to  the  mid-brain.  In  later 
stages  the  opening  of  the  epiphysis  and  the  anterior  boundary  of  the  posterior 
commissure  are  separated  by  a  narrow  band  of  ependyma.  Immediately  in 
front  of  the  epiphysis,  close  to  the  external  surface  of  the  medullary  wall,  is  an- 
other tract  of  nerve-fibers  which  is  very  small  and  is  known  as  the  superior  com- 
missure. The  superior  commissure  develops  much  later  than  the  posterior,  and 
is  much  smaller  in  size.  The  two  commissures  are  found  in  vertebrates  of  all 
classes  and  are  exceedingly  constant  anatomical  features  of  the  brain .  Anterior  to 
the  epiphysis  the  dorsal  roof  of  the  diencephalon  forms  a  broad  arch  which  de- 
scends in  the  figure  vertically  until  it  ends  in  a  small  inward  projection,  Plx,  of  the 
brain-wall,  the  anlage  of  the  choroid  plexus.  Below  this  point  the  brain-wall  is 
continued,  forming  the  lamina  terminalis.  It  then  makes  a  bend  almost  at  right 
angles  and  runs  in  a  horizontal  direction  toward  the  dorsal  side  of  the  embryo. 
This  portion  of  the  brain-wall  shows  a  considerable  thickening,  the  optic  chiasma. 
Behind  the  optic  chiasma  the  brain-wall  forms  a  short  evagination,  the  infundi- 
bular gland,  which  bends  over  so  as  to  lie  close  to  the  dorsal  side  of  the  hypophy- 
sis, Hyp.  The  hypophysis,  which  in  earlier  stages  appears  clearly  as  an  evagina- 
tion of  the  oral  epithelium  (Fig.  123),  is  now  entirely  separated  from  the  mouth, 
and  is  an  epithelial  vesicle  with  an  irregular  cavity.  The  epithelium  has  sent  out, 
especially  on  its  anterior  side,  a  number  of  solid  outgrowths.  The  infundibular 
gland  and  hypophysis  constitute  the  pituitary  body  of  .the  adult.-  They  are  sur- 
rounded by  loose* mesenchymal  tissue.-  The  sella  turcica,  in  which  the  pituitary 
body  of  the  adult  is  lodged,  is  already  marked  out,  because  the  chondrification, 
which  is  to  form  the  sphenoidal  cartilages,  has  already  begun  about  these  struc- 
tures. The  sphenoidal  cartilage  is  continuous,  on  the  one  hand,  with  that  of  the 
nasal  septum,  Sept;  and,  on  the  other,  with  that  of  the  vertebral  column,  Vert. 

FIG.  151. — PIG,"  24.0  MM.     SAGITTAL  SERIES  63,  SECTION  30. 

A,  Arachnoid  space,  in  this  specimen  containing  extravasated  blood.  A.Ao,  Arch  of  the  main  aorta.  All.ar, 
Allantoic  artery.  Ao,  Dorsal  aorta.  Ao.D,  End  of  dorsal  aorta.  Ar.hab,  Habenular  arch  (floor  of  mid- 
brain).  A.vert,  Vertebral  artery  joining  its  mate  to  form  the  basilar  artery.  Bro,  Main  bronchus  of 'lung. 
bro,  Branch  bronchus  within  the  lung.  Cbl,  Cerebellum.  Cce,  Coelom.  Diaph,  Diaphragm.  Dien,  Dien- 
cephalon. Duo,  Duodenum.  Epen,  Ependymal  roof  of  hind-brain.  •  Ephys,  Epiphysis.  G',  Spinal 
ganglion.  Hyp,  Hypophysis.  /«,  Intestine.  Int.v,  Anlage  of  intervertebral  ligament.  La,  Lateral  wall 
of  larynx.  Li,  Liver.  L^l,  Lung.  M.b,  Mid-brain.  Md.ob,  Medulla  oblongata.  •  Nch,  Notochord. 
(E,  Oesophagus.  Pen,  Penis.  P/i,  Pharynx.  Plx,  Choroid  plexus  of  fore-brain.  Plx.  IV,  Choroid  plexus 
of  hind-brain.  Sept,  Cartilaginous  nasal  septum.  Sp.c,  Spinal  cord.  Sp.ren,  Suprarenal  capsule.  St, 
Stomach.  7y,7y/,  Tail.  Te,  Testis.  Ton,  Anterior"  portion  of  tongue.  Umb,  Umbilical  cord.  Vc,  Car- 
dinal vein.  Ven,  Ventricle  of  the  heart.  Ven. IV,  Fourth  ventricle,  or  cavity  of  the  hind-brain.  Vert, 
Vertebra.  W.b,  Wolffian  body.  X  8  diams. 


266  STUDY  OF  PIG  EMBRYOS. 

From  the  opening  of  the  infundibular  gland  the  brain- wall  ascends  and  joins  the 
habenular  arch,  where  it  suddenly  thickens.  The  arch  forms  the  floor  of  the  mid- 
brain.  The  roof  of  the  mid-brain,  M.  b,  is  quite  thin,  and  forms  the  large  arch 
in  which  the  differentiation  of  the  anterior  and  posterior  corpora  quadrigemina 
is  not  yet  shown.  At  its  posterior  boundary  the  wall  of  the  roof  of  the  mid-brain 
bends  inward,  marking  the  constriction  of  the  so-called  isthmus.  We  now  reach 
the  cavity,  Ven.  IV,  or  fourth  ventricle,  of  the  hind-brain.  This  cavity  is  sub- 
divided into  an  anterior  and  a  posterior  portion.  The  boundary  is  marked  on 
the  dorsal  side  by  the  inward  projection  of  the  ependymal  roof  of  the  ventricle 
to  form  the  choroid  plexus,  Plx.  IV,  and  on  the  ventral  side  by  the  angle  formed 
by  the  union  of  the  medulla  oblongata,  Md.  ob,  with  the  vertical  peduncles  of 
the  brain.  In  front  of  the  choroid  plexus  the  arching  brain- wall,  Cbl,  represents 
the  median  anlage  of  the  cerebellum.  The  lateral  portions  of  the  cerebellum  are 
much  thicker.  Behind  the  choroid  plexus  the  roof,  Epen,  of  the  fourth  ventricle 
is  very  thin.  The  medulla  oblongata,  Md.  ob,  is  a  thick  mass  of  tissue  which 
passes  over  abruptly  into  the  spinal  cord.  The  spinal  cord  is  cut,  as  a  whole, 
somewhat  obliquely.  In  its  upper  part,  where  the  reference  line  Sp.  c  is  placed, 
the  section  is  almost  exactly  median,  and  shows,  therefore,  the  floor-plate  or  raphe 
of  the  spinal  cord.  In  front  of  the  cord  is  the  vertebral  artery,  A.  vert,  which 
joins  its  fellow  to  form  the  basilar  artery  which  runs  in  the  median  line  the  entire 
length  of  the  hind-brain.  The  vertebral  column  is  in  the  cartilaginous  stage.  It 
is  an  absolutely  continuous  uninterrupted  rod  of  cartilage  which  merges  at  the 
neck  with  the  cartilaginous  skull.  The  entire  continuous  cartilaginous  structure 
is  termed  the  chondrostyle.  Out  of  it  both  the  cartilaginous  skull  and  the  verte- 
bra are  differentiated.  More  or  less  nearly  in  the  center  of  the  chondrostyle  are 
found  the  remnants  of  the  notochord,  which,  however,  never  extends  anterior  to 
the  pituitary  body,  Hyp.  The  division  of  the  chondrostyle  into  separate  verte- 
brae is  indicated  by  the  modifications  of  the  notochord  and  by  the  commencing 
differentiation  of  the  intervertebral  ligaments.  The  space  occupied  by  the  noto- 
chord expands  in  the  region  corresponding  to  the  division  between  each  two  ver- 
tebrae. The  notochord  in  the  intervertebral  expansions  is  partly  degenerated, 
forming  an  enlarged  mass  of  irregular  strands  of  cells.  From  each  such  mass 
goes  off  a  narrow  extension  of  the  notochord,  through  what  is  to  become  the  body 
of  the  vertebra.  Sometimes  this  extension  is  continuous  with  the  intervertebral 
portions  of  the  notochord,  but  more  usually  it  forms  a  series  of  isolated  fragments, 
for  the  notochord  in  the  parts  corresponding  to  the  bodies  of  the  vertebrae  is 
already  in  process  of  resorption.  The  diameter  of  the  chondrostyle  is  nearly  uni- 
form in  the  vertebral  region,  but  is  a  little  smaller  in  the  part  corresponding  to 
each  body  of  a  vertebra  and  a  little  wider  in  the  parts  corresponding  to  the  inter- 


EMBRYO  OF  24  MM.  267 

vertebral  ligaments.  The  cartilage  of  the  body  of  the  vertebra  continues  past  the 
intervertebral  expansion  of  the  notochordal  cavity,  but  the  external  portion  of 
the  chondrostyle  opposite  each  such  expansion  exhibits  a  modification  of  its  cells, 
for  they  have  become  lengthened  out  in  a  direction  parallel  with  the  vertebral  axis. 
The  tissue  thus  produced  is  the  anlageof  the  vertebral  ligaments.  The  mouth 
and  pharynx,  Ph,  form  a  narrow  cavity,  the  floor  of  which  is  constituted  by  the 
tongue,  Ton,  the  tip  of  which  has  already  become  free.  The  surface  of  the  tongue 
forms  a  long  arch,  at  the  posterior  end  of  which  lies  the  epiglottis,  a  projecting 
fold  of  tissue  which  covers  the  opening  of  the  trachea.  The  side  of  the  trachea  is 
marked  by  the  longitudinal  fold,  La,  which  separates  the  trachea  proper  from  the 
upper  end  of  the  oesophagus,  CE.  At  the  upper  end  of  the  oesophagus  there  is  a 
small  dorsal  diverticulum.  If  the  reference  line  (E  be  followed  a  short  distance 
past  the  oesophagus,  it  will  lead  to  the  section  of  the  main  aorta.  A  little  lower 
down  is  the  section  of  the  arch  of  the  aorta,  A.  Ao.  The  heart  shows  chiefly  its 
large  ventricle,  Ven.  The  section  is  not  favorable  for  an  exhibition  of  its  struc- 
ture or  for  that  of  the  lungs,  Lu.  It  does,  however, — since  in  this  part  of  the 
embryo  the  section  passes  to  one  side  of  the  median  plane, — show  the  main  bron- 
chus, Bro,  coming  off  from  the  trachea  to  the  lung,  and  some  of  the  smaller  ento- 
jlermal  bronchial  branches,  bro,  in  the  lung  itself.  The  heart  and  lung  are  sepa- 
rated from  the  abdominal  cavity  by  the  diaphragm,  Diaph.  It  is  only  to  the 
dorsal  part  of  this  diaphragm  that  the  liver,  Li,  is  attached.  In  earlier  stages 
the  liver  is  connected  with  the  whole  of  the  diaphragm  (septum  transversum) .  We 
now  have  a  portion  of  the  diaphragm  without  connection  with  the  liver.  Below 
the  liver  is  the  section  of  the  stomach,  St,  the  entoderm  of  which  is  cut  twice. 
Below  the  stomach  lies  the  duodenum,  Duo,  extending  from  the  dorsal  side  of  the 
embryo  and  running  toward  the  umbilicus.  At  the  dorsal  end  of  the  duodenum 
is  a  group  of  clusters  of  darkly  stained  cells,  marking  the  position  of  the  anlage 
of  the  pancreas.  Below  the  duodenum  the  loops  of  the  intestine,  In,  are  cut  re- 
peatedly. On  the  dorsal  side  of  these  loops  is  the  section  of  the  genital  gland,  in 
this  specimen,  testis,  Te.  Dorsal  wards  from  the  genital  gland  is  the  complicated 
anlage  of  the  suprarenal  capsule,  Sp.  ren,  which  is  really  a  double  organ,  having 
one  part  derived  from  the  sympathetic  nervous  system  and  another  from  a  modi- 
fication of  mesenchymal  cells.  In  a  sagittal  series  the  connection  of  the  anlage 
with  the  sympathetic  nervous  chain  of  the  abdomen  can  be  readily  made  out- 
In  the  anlage  the  nerve-fibers  and  the  sympathetic  cells  are  irregularly  distrib- 
uted, although  the  cells  are  more  or  less  grouped  together.  The  sympathetic  tissue 
constitutes  the  dorsal  part  of  the  anlage  and  gives  rise  to  the  so-called  medulla  of 
the  adult  organ.  The  ventral  portion  of  the  anlage,  as  seen  in  the  section,  con- 
sists of  bands  or  cords  of  cells  separated  from  one  another  by  venous  sinusoids. 


268  STUDY  OF  PIG  EMBRYOS. 

The  cells  are  much  more  closely  compacted  in  this  portion  of  the  anlage  than  in 
the  sympathetic,  and  they  are  further  distinguished  by  having  nuclei  which  stain 
much  less  deeply.  The  cords  of  cells,  here  seen,  develop  into  the  cortex  of  the 
adult  organ.  The  fate  of  the  medulla  or  sympathetic  portion  of  the  suprarenal 
in  man  is  not  known.  The  section  passes  through  the  side  of  the  allantois,  and, 
therefore,  shows  only  one  of  the  lateral  arteries,  All.  ar,  but  the  allantois  still  bears 
a  number  of  degenerating  mesothelial  villi  (compare  page  222).  At  the  pelvic 
end  of  the  abdomen  a  small  bit  of  the  Wolffian  body,  W.  b,  is  displayed. 


CHAPTER  V. 
STUDY  OF  YOUNG  CHICK  EMBRYOS. 

Method  of  Obtaining  Embryos. 

Fertile  eggs  can  usually  be  obtained  from  dealers,  who  can  supply  them  in 
quantities  as  needed,  or  hens  may  be  kept  with  little  trouble  especially  for  the 
purpose.  In  that  case  the  hen  herself  will  be  found  the  best  incubator,  for  the 
number  of  eggs  which  develop  normally  under  a  hen  is  larger  than  in  an  artificial 
incubator,  and  abnormalities  of  development  are  less  frequent.  A  good  setter 
will  remain  upon  the  eggs,  even  though  some  are  removed  and  replaced  by  fresh 
ones,  for  about  a  month.  She  should  be  plentifully  supplied  with  water  and  soft 
food,  which  is  best  kept  at  a  little  distance  off,  so  that  she  will  be  obliged  to 
leave  the  eggs  to  feed.  A  box  that  is  somewhat  secluded,  and  affords  some 
protection,  warmth,  and  shelter  from  the  light,  should  be  provided.  In  order  to 
obtain  the  most  accurate  results  it  is  desirable  fo  place  the  eggs  as  soon  as  laid 
immediately  under  the  hen.  Only  by  this  means  can  an  approximate  correla- 
tion between  the  stage  of  development  and  the  duration  of  incubation  be  secured. 

Artificial  incubators  are  now  made  to  work  satisfactorily.*  The  tem- 
perature of  an  incubator  should  be  maintained  at  about  38°  C.  (100.4°  F.).  It 
should  on  no  account  be  allowed  to  rise  above  40°  C.  (104°  F.),  for  that  destroys 
a  portion  of  the  eggs  and  causes  the  production  of  many  abnormalities  in  the 
remainder;  and,  if  possible,  a  fall  to  a  lower  temperature  should  be  avoided, 
although  the  results  of  a  lower  temperature  are  less  disastrous.  No  incubator 
should  be  used  which  does  not  permit  a  constant  supply  of  fresh  air  and  of  mois- 
ture. The  date  should  always  be  marked  on  each  egg  when  it  is  placed  in  the 
incubator.  If  a  number  of  eggs  from  a  dealer  are  artificially  incubated  the  same 
length  of  time,  they  are  pretty  sure  to  cover  a  considerable  range  of  stages,  as,  of 
course,  eggs  so  supplied  are  of  varying  ages,  the  exact  time  of  laying  not  being 
recorded. 

*  The  one  used  at  the  Harvard  Medical  School  is  heated  by  a  kerosene  lamp  and  has  a  capacity  of  100 
eggs,  ft  is  called  the  Plymouth  Incubator,  and  is  sold  by  Charles  I.  Nesmith,  Reading,  Mass.  In  the  market 
other  incubators  may  be  found,  doubtless  equally  good,  among  them  patterns  adapted  for  the  use  of  gas,  where 
that  is  preferred. 

269 


270  STUDY  OF  YOUNG  CHICK  EMBRYOS. 

In  this  work  two  stages  of  the  chick  are  especially  studied.  The  first  is  nor- 
mally produced  after  about  forty-six  hours'  incubation.  The  embryo  should 
have  about  twenty-eight  segments  and  three  gill  clefts  showing  externally.  Em- 
bryos a  little  less  or  a  little  more  developed  are  almost  equally  serviceable.  The 
second  stage  studied  is  that  of  a  chick  with  seven  segments,  which  is  normal 
after  about  twenty-seven  hours'  incubation.  The  student  will  find  it  advan- 
tageous to  begin  his  studies  with  older  stages,  as  these  can  be  more  easily  manip- 
ulated. 

Removing  the  Embryo. — Before  the  egg  is  opened  a  basin  should  be  prepared 
and  filled  with  normal  salt  solution  warmed  to  about  40°  C.  (104°  F.),.  The 
basin  should  be  large  enough  to  permit  the  entire  egg  to  be  submerged  in  it. 

Take  the  egg  warm  from  the  incubator  or  the  hen ;  allow  it  to  rest  quietly  in 
one  position  for  two  or  three  minutes  before  opening  it.  This  is  in  order  to  insure 
that  the  side  of  the  yolk  which  contains  the  embryo  is  turned  uppermost.  After 
an  egg  is  disturbed  the  yolk  will  turn  and  resume  its  normal  position,  for  which 
but  a  short  time  is  necessary.  The  egg  may  now  be  held  in  one  hand,  the  shell 
cracked  and  the  pieces  of  the  shell  above  the  yolk  be  removed  with  forceps,  mak- 
ing a  hole  about  an  inch  in  diameter.  The  inner  egg  membrane  may  be  removed 
with  the  shell.  If  any  of  the  white  of  the  egg  tends  to  overflow,  it  should  be 
immediately  snipped  off  with  a  pair  of  scissors,  otherwise  it  will  cause  the  yolk 
to  roll  over,  thus  concealing  the  embryo. 

The  embryo  and  germinal  area  are  now  to  be  examined  with  the  naked  eye, 
or,  better,  with  a  hand  lens.  The  student  will  detect  very  easily  the  area  pellu- 
cida,  which  lies  at  right  angles  to  the  long  axis  of  the  egg,  and  also  see  in  the  middle 
of  the  area  a  long  whitish  streak,  which  marks  the  anlage  of  the  embryo.  Around 
the  area  pellucida  can  be  seen  the  mottled  vascular  area  which  will  vary  in  ap- 
pearance according  as  the  development  of  the  blood-vessels  and  blood-islands  is 
more  or  less  advanced.  The  area  vasculosa  is  a  portion  of  the  larger  area  opaca 
which  merges  at  its  periphery  into  the  general  yolk.  In  embryos  of  the  second 
half  of  the  second  day,  thirty-six  to  forty-eight  hours,  the  contraction  of  the  heart 
can  be  readily  seen,  and  usually  the  outlines  of  the  head  of  the  embryo  may  be 
made  out.  The  germinal  area  is  now  to  be  separated  from  the  rest  of  the  yolk. 
To  accomplish  this,  plunge  one  blade  of  a  pair  of  sharp  scissors  into  the  yolk  a 
little  beyond  the  edge  of  the  vascular  area,  and  cut  rapidly  around  until  a  cir- 
cular incision  has  been  completed ;  then  take  a  flat  spatula  and  plunge  it  boldly 
into  the  yolk  at  a  depth  of  perhaps  an  eighth  of  an  inch  underneath  the  embryo. 
Next  lift  out  the  embryo  together  with  the  yolk  and  the  overlying  white  of  the 
egg,  steady  it  a  little  if  necessary  on  the  spatula  with  a  pair  of  forceps  or  needle, 
and  transfer  it  rapidly  to  the  dish  of  warm  salt  solution.  With  a  pair  of  fine 


METHOD  OF  OBTAINING  EMBRYOS.  271 

forceps  the  edge  of  the  germinal  area  may  be  seized,  and  by  gentle  motion  it  may 
be  separated  from  the  mass  of  yolk  and  also  from  the  thin,  whitish,  overlying 
membrane  of  the  yolk,  and  at  the  same  time  from  so  much  of  the  white  of  the  egg 
as  may  have  been  carried  along.  As  one  becomes  more  practised  in  these  opera- 
tions, it  is  not  difficult  to  remove  the  germinal  area  without  taking  much  yolk 
along  with  it. 

The  operation  may  be  modified  as  follows :  After  the  shell  is  opened  the  egg 
may  be  tilted  so  as  to  allow  the  white  to  run  off,  and  as  it  runs  over  the  edge  it  is 
snipped  through  with  the  scissors,  and  as  much  of  the  white  removed  as  is  possible 
in  this  way.  The  whole  egg  is  then  submerged  in  the  warm  salt  solution,  an  inci- 
sion around  the  germinal  area  made  as  above,  and  the  embryo  floated  off  from 
the  yolk. 

Preservation  of  the  Embryo. — The  next  step,  after  the  embryo  has  been  re- 
moved from  the  yolk  and  lies  in  the  salt  solution,  is  to  put  a  glass  slide  in  the  salt 
solution  and  carefully  float  the  embryo  and  germinal  area  upon  it,  and  then  re- 
move them  together.  The  slide  is  now  to  be  laid  flat  on  the  table  and  the  germinal 
area  spread  out  carefully  upon  it.  In  this  operation  good  results  may  often  be 
obtained  by  allowing  a  few  drops  of  warm  salt  solution  to  fall  upon  the  center  of 
the  germinal  area.  The  currents  produced  by  the  falling  drops  will  be  sufficient 
to  spread  out  the  blastoderm  in  its  natural  form,*  and  at  the  same  time  to  wash 
away  any  superfluous  yolk  grains  that  may  be  adherent  to  the  preparation.  At 
this  stage  the  preparation  should  be  examined  by  the  student  with  a  low  power  of 
the  microscope,  as  described  below.  To  preserve  the  specimen,  four  or  five  drops 
of  Zenker's  fluid  are  allowed  to  fall  upon  the  specimen  gently  and  quietly  as  it 
lies  upon  the  glass  slide.  The  specimen  is  allowed  to  stand  for  about  ten  minutes 
'and  is  then  transferred  to  a  dish  containing  a  larger  quantity  of  Zenker's  fluid. 
The  transfer  should  be  made  by  submerging  one  end  of  the  slide  in  the  dish  and 
floating  the  specimen  off.  In  from  two  to  four  hours  the  hardening  of  the  speci- 
mens will  be  completed.  They  must  then  be  washed  thoroughly  by  decanting 
off  the  Zenker's  fluid  and  replacing  it  with  water,  and  this  water  must  itself  be 
replaced  several  times  during  the  next  twenty-four  hours.  Further  treatment 
of  the  specimen  is  as  described  on  page  363. 

The  Making  of  Serial  Sections. — Specimens  are  best  colored  with  alum  cochi- 
neal in  toto.  They  are  then  imbedded  in  paraffin  and  cut  into  series.  The  most 
useful  sections  are  those  which  are  transverse  to  the  axis  of  the  spinal  cord.  They 
should  not  exceed  10  //  in  thickness. 

*  The  student  will  observe  that  the  fresh  blastoderm  is  very  easily  distorted. 


272 


STUDY  OF  YOUNG  CHICK  EMBRYOS. 


Embryo  Chick  with  about  Twenty=four  Segments  and  Three  Gill  Clefts  (about 

Forty-six  Hours'  Incubation). 

The  following  description  will  apply  almost  equally  well  to  embryos  with 
from  twenty-six  to  twenty-nine  segments. 

Examination  in  Toto. — The  specimen  as  a  whole,  as  in  the  fresh  state,  has  a 


M.b. 


Pr.g. 


FIG.  152. — EMBRYO  CHICK  WITH  ABOUT  TWENTY-FOUR  SEGMENTS.    SURFACE  VIEW  FROM  THE  DORSAL  SIDE. 

A.c.v,  Amnio-cardiac  vesicle.  Am./,  Posterior  edge  of  the  amniotic  fold.  A.o,  Area  opaca.  A.p,  Area  pel- 
lucida.  A.vi,  Arteria  vitellina  (or  omphalo-mesaraica).  bl.is,  Blood-island  in  the  area  pellucida.  Ht, 
Heart.  M.b,  Mid-brain.  Md,  Medullary  canal  or  spinal  cord.  Op,  Optic  vesicle.  Ot,  Otocyst.  P>'-g, 
Primitive  groove.  Seg,  Primitive  segment.  Seg.z,  Segmental  zone.  X  IS  diams. 

grayish  tint  when  viewed  by  transmitted  light.  As  soon  as  it  is  hardened  the 
opacity  of  all  the  tissues  is  greatly  increased.  In  the  center  of  the  germinal  area  is 
the  very  conspicuous  area  pellucida ,  which  is  somewhat  pear-shaped.  The  por- 
tion around  the  anterior  end  of  the  embryo  (Fig.  152),  A.p,  is  very  wide.  In  the 


<  J  V    ' 

EMBRYO   WITH  TfyENTY-FOUd  SEGMENTS. 


273 


center  of  the  area  vasculosa  appears  tl^e  embryo*  "1»he  head  end  of  whiclLJs  twisted 
over  so  that  the  left  side  of  the  head  lies  against  the  yolk.  This  twiltin^  of  the 
neck  and  head  so  that  they  become  asymmetrical  in  position  is  very  characteristic 
of  birds.  Below  the  head  and  somewhat  to  the  right  may  be  Seen  ^he  £\ibular 
heart,  Ht,  which,  in  the  fresh  specimen,  pulsates  regularly.  Around  ^e  area 
pellucida  comes  the  dark  area  opaca,  in  which  we  readily  distinguish  the'  outer 
boundary,  or  terminal  sinus,  of  the  area  vasculosa.  In  this  tljere  is  afready  a 
well-developed  network  of  blood-vessels  through  which  the  blflod  is  circulating, 
being  driven  by  the  heart.  The  blood  moves  out  from  the  embryo  by  two  large 
vessels,  A.  m,  which  lie  symmetrically,  the  vitelline  or  omphalo-mesaraic  ar- 
teries. These  arteries  arise  from  the  dorsal  aorta  of  the  embryo  and  pass  out  to 
the  area  vasculosa,  over  which  they  ramify.  The  blood  returns  'to  the  heart  by 
means  of  the  omphalo-mesaraic  veins,  of  which  the  anterior  branches  *  are  alone 
clearly  differentiated  at  this  stage.  The  general  form  of  the  embryo  is  indicated 
by  figure  152.  In  the  region  of  the  head  we  notice  the  very  well-marked  head- 
bend  which  is  established  in  the  region  of  the  mid-brain,  M.  b.  The  medullary 
tube  in  the  region  of  the  head  is  very  much  enlarged  and  is  divided  into  three 
well-marked  primary  cerebral  vesicles.  The  first  of  these  is  quite  large,  and  at 
its  side  lies  the  anlage  of  the  eye,  Op,  in  the  center  of  which  one  readily  distin- 
guishes the  commencement  of  the  lens.  The  second  cerebral  vesicle  is  much 
smaller  than  the  first  in  every  dimension.  It  occupies  the  region  of  the  head- 
bend  and  is  separated  from  the  first  vesicle  by  a  constriction,  and  from  the  third 
vesicle  by  another  constriction.  The  third  vesicle  in  length  more  than  equals 
the  first  and  second  combined,  and  at  its  widest  part  is  nearly  equal  in  diameter 
to  the  second  vesicle.  It  tapers  out  toward  the  caudal  end  of  the  embryo  and 
passes  over  into  the  much  smaller  portion  of  the  medullary  canal,  which  repre- 
sents the  anlage  of  the  spinal  cord.  At  the  side  of  the  third  vesicle  we  can  see 
the  beginning  of  the  formation  of  the  ear  or  otocyst,  Ot.  On  the  side  of  the  neck 
between  the  third  cerebral  vesicle  and  the  heart  there  are  three  external  depres- 
sions which  bound  the  first  and  second  branchial  arches,  1,2,  of  the  embryo. 
Behind  each  arch  the  depression  marks  the  site  of  a  gill  cleft.  The  first  is  the 
longer,  the  second  the  shorter.  Between  the  projecting  head  and  the  first  bran- 
chial arch  the  outline  of  the  embryo  makes  a  depression,  which  marks  the  posi- 
tion of  the  developing  oral  cavity.  The  heart  is  a  large  tube,  Ht.  The  omphalo- 
mesaraic  veins  join  the  venous  or  posterior  end  of  the  heart.  The  heart  is  very 
much  bent  ;  its  anterior  end  turns  toward  the  gill  clefts  and  there  gives  off  the 
primitive  aortic  branches,  which  finally  join  again  so  as  to  form  the  median  dorsal 

*  The  vessels  do  not  appear  in  the  figure. 
18 


274  STUDY  OF  YOUNG  ClflCK  EMBRYOS. 

aorta  which  sends  off  the  two  vitelline  arteries,  A.  m.  On  either  side  of  the  me- 
dullary canal  can  be  seen  the  primitive  segments,  Seg.  The  first  of  these  which 
is  distinct  lies  close  behind  the  otocyst.  At  the  posterior  end  of  the  embryo  addi- 
tional segments  are  still  forming,  and  the  precise  number  of  segments  varies  from 
embryo  to  embryo.  The  medullary  canal,  Md,  is  closed,  but  beyond  its  extreme 
limit  traces  of  the  primitive  groove,  Pr.  g,  can  still  be  seen.  The  network  cf 
blood-vessels  over  the  area  vasculosa  is  very  distinct  and  characteristic.  .The 
network,  however,  does  not  yet  extend  into  the  body  of  the  embryo  proper.  The 
limit  of  the  body  of  the  embryo  is  suggested  by  the  darker  tissue,  Seg.  z,  surround- 
ing the  spinal  cord,  Md,  on  either  side.  About  the  hinder  end  of  the  embryo,  both 
in  the  pellucida  and  in  the  opaca,  appear  a  number  of  small  spots,  the  blood- 
islands,  bl.  is,  many  of  which  have  in  the  fresh  specimen  a  reddish  color.  In 
hardened  specimens  the  opacity  of  the  blood-islands  renders  them  conspicuous, 
especially  in  the  area  pellucida. 

Embryo  Chick  with  Twenty=eight  Segments. 

The  Study  of  Transverse  Sections. — A  series  of  figures  from  transverse  sec- 
tions of  an  embryo  of  this  stage  is  herewith  presented.  They  have  been  selected 
so  as  to  show  the  principal  typical  structures.  The  position  of  the  sections  can 
be  followed  more  easily  by  comparing  each  transverse  section  with  figure  166, 
to  determine  its  place  and  the  organs  through  which  it  must  pass. 

Section  through  the  Right  Auditory  Invagination  (Fig.  153). — Owing  to  a 
curvature  of  the  neck-bend  of  the  head  the  section  is  not  symmetrical.  It  passes 
through  both  the  hind-brain,  h.  b,  and  the  fore-brain,  /.  b.  Underneath  the 
former  appears  a  small  structure,  nch,  the  notochord,  and  on  one  side  can  be  seen 
the  auditory  invagination,  Ot,  which  is  formed  wholly  by  the  locally  thickened 
ectoderm,  which  is  elsewhere  quite  thin.  The  ectoderm,  EC,  covering  the  dorsal 
side  of  the  hind-brain  is  very  thin,  but  the  portion  in  front  of  the  auditory  invagi- 
nation is  somewhat  thicker.  The  ectoderm  of  the  invagination  is  very  much 
thickened  and  contains  numerous  somewhat  crowded  nuclei  at  all  levels.  These 
nuclei  are  rounded  in  form  and  have  one  or  two  very  distinct  nucleoli.  On  the 
posterior  side  of  the  otocyst  there  is  very  little  mesoderm;  on  the  anterior  side, 
much  more.  Between  the  developing  otocyst  and  the  notochord  there  is  a  blood- 
vessel, ve,  with  merely  endothelial  walls,  a  branch  of  the  cardinal  vein.  Between 
the  hind-brain  and  fore-brain  near  the  notochord,  the  two  aortae  Ao,  are  cut.  In 
their  interior  there  can  usually  be  seen  a  certain  number  of  nucleated  cells  vary- 
ing somewhat  in  size  and  appearance,  but  generally  having  a  rounded  form  with 
distinct  outline  and  a  well-defined  nucleolated  nucleus. 

Section  through  the  Left  Auditory  Invagination  (Fig.  154). — Owing  to  the 


EMBRYO   WITH  TWENTY-EIGHT  SEGMENTS. 


275 


irregular  form  of  the  embryo  the  sections  through  the  otocyst  are  not  symmetrical. 
The  present  section  shows  the  opening  of  the  left  otocyst,  Ot.s,  and  a  closed  section 
of  the  right  otocyst,  Ot.  d.  At  its  lower  inner  edge  the  outer  boundary  of  the  wall 
of  the  otocyst  is  indistinct,  this  appearance  being  due  to  the  union  of  the  cells  of 
the  acoustic  ganglion  with  the  wall  of  the  otocyst.  The  section  also  passes  through 
the  first  gill  cleft,  cl.  i,  of  the  right  side,  and  shows  very  distinctly  indeed  the 


Epen. 


h.b. 


Ot. 


Nth. 


h.b. 


Ot.d. 


Re. 


Ao.  D. 


FIG.  153. — SECTION  OF  CHICK  EMBRYO  WITH 
ABOUT  TWENTY-EIGHT  SEGMENTS.  TRANS- 
VERSE SERIES  92,  SECTION  73. 

Ao,  Aorta.  EC,  Ectoderm.  F.b,  Fore-brain.  h.b, 
Hind-brain,  tries,  Mesoderm.  Nch,  Notochord. 
Of,  Otocyst.  Ve,  Vein.  X  5°  diams. 


FJG.  154. — SECTION  OF  CHICK  EMBRYO  WITH 
ABOUT  TWENTY-EIGHT  SEGMENTS.  TRANS- 
VERSE SERIES  92,  SECTION  83. 

Ao.D,  Descending  aorta.  Ao.2,  Second  aortic  arch. 
card,  Anterior  cardinal  vein.  cl.pl,  Closing 
plate,  cl.i,  First  gill  pouch.  EC,  Ectoderm. 
Epen,  Roof  of  hind-brain,  f.b,  Fore-brain,  h.b, 
Hind-brain,  mes,  Mesoderm.  nch,  Notochord. 
Op,  Optic  vesicle.  Ot.d,  Right  otocyst.  Ot.s, 
Left  otocyst.  PA,  Pharynx.  X  5°  diams. 

closing  plate,  cl.  pi,  which  is  formed  by  a  fusion  of  the  ectodermal  and  entodermal 
cells.  On  the  opposite  side  of  the  section  the  same  cleft  is  imperfectly  shown.  On 
the  posterior  side  of  the  cleft  is  the  second  aortic  arch,  Ao.  2,  and  on  the  ante- 
rior side  of  the  cleft,  extending  toward  the  fore-brain,  /.  b,  appear  the  sections  of 
the  two  descending  aortae,  Ao.  D.  The  part  shown  in  this  section  is  that  which 
connects  the  dorsal  ends  of  the  first  and  second  aortic  arches.  In  the 


276 


STUDY  OF  YOUNG  CHICK  EMBRYOS. 


Epen. 


h.b. 


card. 


-    cl.I. 


Mdb. 


Ent. 


Ret. 


region  of  the  fore-brain  appears  a  shaving  from  the  edge  of  the  optic  evagination, 
Op.  The  anterior  cardinal  veins,  card,  appear  just  inside  of  the  otocyst  close  to 
the  ventral  wall  of  the  hind-brain,  h.  b. 

Section  through  the  Imagination  of  the  Optic  Lens  (Fig.  155). — This  section 
also  passes  through  the  hind-brain,  h.  b,  fore-brain,  /.  b,  and  through  the  openings 

of  both  invaginations  to  form  the  lens,  L, 
which  bears  a  striking  resemblance  to 
the  invagination  which  forms  the  oto- 
cyst. The  ectoderm,  EC,  over  the  roof  of 
•the  fore-brain  is  very  thin  and  passes 
abruptly  into  the  thickened  layer  which 
forms  the  wall  of  the  invagination.  On 
the  ventral  side  the  ectoderm  is  some- 
what thicker.  The  wall  of  the  lentic 
vesicle  is  quite  thick,  its  nuclei  *are 
numerous,  but  are  situated  chiefly  on 
the  mesodermal  side  of  the  layer;  so 
that  toward  its  outer  surface  the  layer 
is  comparatively  free  from  nuclei.  The 
invagination  of  the  lens  rests  against 
the  optic  vesicle,  the  wall  of  which,  Ret, 
next  to  the  lens  is  thicker  than  the  pos- 
terior or  inner  wall  of  the  optic  vesicle. 
The  thickened  outer  portion  is  the  an- 
lage  of  the  retina,  the  thinner  inner 
portion  is  the  anlage  of  the  pigment 
layer  of  the  retina.  The  fore-brain, 
/.  b,  has  an  elongated  form  with  quite 
thick  walls  crowded  with  nuclei.  Be- 
tween it  and  the  hind-brain  appears  the 
cavity  of  the  pharynx,  Ph,  which  on  the 
left  side  of  the  embryo  shows  a  prolon- 
gation, cl.  I ,  which  extends  almost  to 
the  surface  of  the  embryo.  This  prolon- 
gation is  the  first  gill  pouch.  On  the 

dorsal  side  of  the  pharynx  appear  the  two  large  aortic  trunks,  Ao.  D,  and  on  its 
ventral  side  the  two  smaller  first  aortic  arches,  Ao.  /.  These  are  situated  in  the 
mandibular  branchial  arch,  Mdb,  which  is  well  marked  externally  by  a  rounded 
protuberance.  The  second  gill  pouch  is  shown  on  the  left-hand  side  of  the  sec- 
tion, cl.  II. 


FIG.  155. — SECTION  OF  CHICK  EMBRYO  WITH 
ABOUT  TWENTY-EIGHT  SEGMENTS.  TRANS- 
VERSE SERIES  92,  SECTION  96. 

Ao.D,  Descending  aorta.  Ao.i,  First  aortic  arch. 
Ao.2,  Second  aortic  arch,  card,  Anterior  car- 
dinal vein.  cl.  I,  First  gill  cleft?  cl.Il,  Second 
entodermal  gill  cleft.  EC,  Ectoderm.  Ent, 
Entoderm.  Epen,  Roof  of  hind-brain,  f.b, 
Fore-brain,  h.b,  Hind-brain.  L,  Invagination 
of  lens.  Mdb,  Mandibular  arch,  mes,  Meso- 
derm.  nch,  Notochord.  Op,  Optic  vesicle. 
Ph,  Pharynx.  Ret,  Retina.  X  5°  diams. 


EMBRYO   WITH  TWENTY-EIGHT  SEGMENTS. 


277 


Ao.D. 


h.b. 


card. 


-  nch. 


cl.  II. 


Section  through  the  Optic  Stalks  (Fig.  1 56). — The  head  of  the  embryo  now  ap- 
pears quite  isolated  from  the  body.  It  is  bounded  by  a  distinct  layer  of  ectoderm, 
EC,  and  contains  the  very  large  fore-brain,  f.b,  which  gives  off  on  either  side  an  optic 
evagination,  Op,  the  walls  of  which  are  quite  thick,  about  the  same  as  those  of  the 
fore-brain  proper.  Each  optic  evagination  is  widest  toward  the  side  of  the  head 
and  is  constricted  toward  the  brain,  with 
which,  therefore,  it  is  connected  by  a  stalk 
in  which  we  can  already  recognize  the  an- 
lage  of  the  optic  nerve.  Between  the  two 
optic  stalks  on  the  side  toward  the  pharynx 
the  floor  of  the  fore-brain  bends  downward 
and  almost  joins  the  superficial  ectoderm. 
All  of  the  space  between  the  walls  of  the 
fore-brain  and  the  optic  evagination-  on 
the  one  hand,  and  of  the  superficial  ecto- 
derm of  the  head  on  the  other,  is  filled 
with  undifferentiated'  mesenchyma.  In 
this  tissue  blood-vessels,  nerves,  lymph- 
atics, and  muscles  will  grow,  and  the  tis- 
sue itself  is  to  produce  the  cutis,  the  sub- 
cutaneous tissue,  the  skull,  the  dura  mater, 
arachnoid  membrane,  and  pia  mater.  We 
have  in  the  present  undifferentiated  stage 
of  this  mesenchyma  a  most  striking  con- 
trast with  the  complicated  histological 
conditions  of  •  the  adult.  The  opposite 
part  of  the  embryo  represents  the  cervical 
region.  At  one  side  we  see  a  small  piece 
of  the  heart  appearing,  Ht,  and  higher  up 
is  the  wide  pharynx,  Ph,  underneath  which 
is  a  blood-vessel,  Ao,  the  main  aorta.  To 
the  left  appears  another  blood-vessel,  Ao. 
2,  a  portion  of  the  second  aortic  arch. 
The  pharynx  shows  on  one  side  the  pro- 
longation of  its  cavity  which  constitutes  the  second  gill  pouch,  cl.  II.  On  the 
dorsal  side  of  the  pharynx  appear  the  descending  aortae,  Ao.  D,  that  on  the  right 
of  the  figure  being  joined  by  the  third  aortic  arch,  near  which  appears  an  accumu- 
lation of  more  deeply  colored  cells,  cl.  II,  part  of  the  entodermal  wall  of  the  second 
gill  pouch.  Between  the  pharynx  and  the  hind-brain  we  have  a  round  section  of 


EC. 


f.b. 


FIG.  156. — SECTION  OF  CHICK  EMBRYO  WITH 
ABOUT  TWENTY-EIGHT  SEGMENTS.  TRANS- 
VERSE SERIES  92,  SECTION  104. 

Ao,  Trunk  of  the  aorta.  Ao.fo,  Descending  aorta. 
Ao.2,  Second  aortic  arch,  card,  Anterior 
cardinal  vein.  cl.II,  Second  entodermal  gill 
pouch.  EC,  Ectoderm,  f.b,  Fore-brain. 
h.b,  Hind-brain.  Ht,  Heart,  mes,  Meso- 
derm.  My,  Muscle  plate,  nch,  Notochord. 
Op,  Optic  vesicle.  Ph,  Pharynx.  X  5° 
diams. 


278  STUDY  OF  YOUNG  CHICK  EMBRYOS. 

the  small  notochord  which  appears  quite  deeply  stained,  and  therefore  stands  out 
conspicuously  from  the  very  loose  mesenchyma  by  which  it  is  surrounded.  It  is 
not  until  later  stages  that  the  mesenchymal  cells  begin  to  crowd  around  the  noto- 
chord to  constitute  the  anlage  of  the  future  vertebral  column.  At  the  present 
stage  the  differentiation  of  the  axial  skeleton  around  the  notochord  has  not  be- 
gun. As  regards  the  hind-brain,  h.  b,  we  know  that  at  its  sides  it  is  already  con- 
siderably thickened,  but  at  its  dorsal  wall  it  is  quite  thin  and  has  already  ex- 
panded considerably,  thus  initiating  the  formation  of  the  thin  ependymal  roof 
of  the  fourth  ventricle.  On  either  side  of  the  hind-brain  appears  a  blood-vessel, 
card,  the  anterior  cardinal,  which  by  transformation  and  migration  is  to  lead  to 
the  formation  of  the  jugular  veins  of  the  adult. 

Section  through  the  Aortic  End  of  the  Heart  (Fig.  157). — The  cervical  region  of 
the  head  and  the  tip  end  of  the  region  of  the  fore-brain  are  cut  separately.  On 
the  lower  side  of  the  pharynx  is  attached  the  double  heart- tube,  of  which  the 
endothelial  portion,  endo,  is  in  actual  contact  with  the  thick  entoderm,  En,  which 
forms  the  floor  of  the  pharynx.  The  heart-tube  shows  its  bend  toward  the  right  of 
the  embryo.  There  is  a  considerable  space  between  the  endothelial  heart  and  the 
muscular  heart,  m.  hi,  and  this  space  is  almost  wholly  free  of  tissue,  except  in  the 
immediate  neighborhood  of  the  pharynx  itself.  Close  to  the  connection  of  the 
heart-tube  with  the  pharyngeal  floor  there  runs  off  on  either  side  the  membrane 
of  the  amnion.  Where  it  starts  from  the  embryo  the  amnion  has  considerable 
thickness  and  appears  somewhat  folded  in  the  section;  but  as  it  turns  to  cover 
the  embryo  it  becomes  very  thin.  It  consists  only  of  two  very  delicate  layers, 
mesodermic  and  ectodermic,  both  one  cell  thick.  The  two  layers  lie  close  to- 
gether, but  are  easily  distinguished.  On  the  right-hand  side  of  the  embryo  the 
raphe  of  the  amnion  may  be  observed,  raph,  and  in  this  section  it  is  constituted 
by  only  two  strands  of  mesoderm  which  pass  over  from  the  amnion  on  to  the 
chorion,  Cho,  or  membrana  serosa,  as  it  has  been  called  by  many  embryologists. 
The  arrangement  of  the  envelopes  of  the  head  is  somewhat  more  complicated. 
Underneath  the  right  *  of  the  section  of  the  cervical  portion  of  the  head  runs  the 
splanchnopleure,  Spl,  in  which  one  can  readily  distinguish  numerous  sections  of 
blood-vessels,  which,  on  the  side  toward  the  embryo,  are  covered  by  mesoderm, 
and  on  the  side  away  from  the  embryo  are  covered  by  entoderm.  If  we  follow 
along  the  splanchnopleure  to  a  point  near  the  section  of  the  region  of  the  fore- 
brain,  we  find  that  it  encounters  a  circle  of  ectoderm,  EC,  which  surrounds  that 
portion  of  the  head.  When  the  splanchnopleure  reaches  this  ectoderm,  its  two 
layers  divide  or  split  apart.  The  mesoderm  bends  off  to  the  right  f  and  forms, 

*  This  means  the  left  side  of  the  embryo.  f  The  right  of  the  embryo, — the  left-hand  side  of  the  figure. 


EMBRYO   WITH  TWENTY-EIGHT  SEGMENTS. 


279 


together  with  a  portion  of  the  ectocferm,  a  part  of  the  true  amnion,  Am' ,  of  the 
head.  The  entoderm,  Ent,  on  the  contrary,  bends  to  the  left  and  joins  the  ecto- 
derm on  that  side  of  the  head  to  form  the  pro-amnibn,  Pro.  am.  Beyond  the 
head  the  entoderm  and  mesoderm  again  unite  and  we  have  a  continuation  of  the 


Md. 


d.lll. 


Pro. am. 


Ent. 


FIG.  157. — SECTION  OF  CHICK  EMBRYO  WITH  ABOUT  TWENTY-EIGHT  SEGMENTS.  TRANSVERSE  SERIES  92, 

SECTION  114. 

Am,  Am',  Amnion.  Ao.D,  Descending  aorta.  Ao.2,  Second  aortic  arch.  Cho,  Chorion.  cl.  If,  Second  ento- 
dermal  gill  pouch.  cl.III,  Third  entodermal  gill  pouch.  EC,  Ectoderm.  En,  Entoderm  of  pharynx. 
endo,  Endothelial  heart.  Ent,  Entoderm  of  pro-amnion.  f.b,  Fore-brain.  Md,  Medulla  obi ongata.  mes, 
Mesoderm  of  amnion.  m.ht,  Muscular  heart,  nek,  Notochord.  Pro. am,  Pro-amnion.  raph,  Raphe  of 
amnion.  Seg,  Segment.  Spl,  Splanchnopleure.  Ve,  Anterior  cardinal  vein.  X  5°  diams. 

splanchnopleure,  Spl.  Owing  to  the  development  of  the  pro-amnion,  the  rela- 
tions of  the  foetal  envelopes  surrounding  the  head  are  complicated.  The  student 
may,  however,  easily  satisfy  himself  that  the  layer,  EC,  in  figure  157,  is  really 


280  STUDY  OF  YOUNG  CJfJCK  EMBRYOS. 

ectoderm  by  following  it  through  in  the  se'ries  of  sections,  for  he  will  then  find 
that  it  becomes  continuous  in  other  regions,  on  the  one  hand,  with  the  ectoderm 
of  the  true  amnion,  and,  on  the  other,  with  the  epidermis  of  the  body  proper.  In 
the  cervical  region  we  have  a  transverse  section  of  the  lower  portion  of  the  hind- 
brain,  Md,  corresponding  to  the  part  of  the  future  medulla  oblongata  near  its 
junction  to  the  spinal  cord.  Underneath  it  is  the  section  of  the  notochord,  nch, 
and  on  either  side  sections  of  a  secondary  segment,  Seg.  Just  below  the  seg- 
ments is  the  cardinal  vein,  Ve,  and  below  the  vein,  but  nearer  to  the  median  line, 
lies  the  dorsal  aorta,  Ao.  D.  The  pharynx  expands  on  each  side;  the  prolonga- 
tion on  the  left  of  the  embryo  is  the  second  gill  pouch,  cl.  II,  that  on  the  right  is 
the  third  gill  pouch.  The  pharynx  itself  is  lined  by  entoderm,  En,  which  is  very 
thin  in  the  median  dorsal  line,  but  immediately  below  the  dorsal  aortae  it  thickens 
abruptly  and  continues  as  a  quite  thick  layer  on  to  the  ventral  side.  In  the 
median  ventral  line  it  forms  a  deep  groove,  and  in  the  walls  of  this  groove  we  find 
that  the  nuclei  are  not  distributed  through  the  whole  thickness  of  the  entoderm, 
but  occupy  chiefly  its  outer  or  basal  portions,  so  that  the  portion  of  the  layer  next 
the  cavity  of  the  groove  is  formed  almost  wholly  of  protoplasm.  At  the  tip  of 
the  gill  pouch  the  entoderm  has  come  into  actual  contact  with  the  ectoderm,  and 
the  cells  of  the  two  germ-layers  have  there  united,  without  distinguishable  boun- 
dary being  kept  between  the  layers.  The  fused  ectoderm  and  entoderm  consti- 
tute the  closing  plate  of  the  gill  cleft,  and  such  a  plate  is  formed  at  the  tip  of  every 
gill  pouch.  On  either  side  of  the  ventral  surface  of  the  pharynx  appears  the  sec- 
tion of  the  second  aortic  arch,  Ao.  2.  By  following  along  through  a  few  sections 
(in  the  series  here  studied,  from  four  to  six)  the  junction  of  these  arches  with  the 
endothelial  tube  of  the  heart  may  be  observed.  The  student  should  verify  this 
connection  and  satisfy  himself  that  the  endothelium  of  the  blood-vessels  is  a  con- 
tinuation of  the  endothelium  of  the  heart.  This  fact  is  of  great  morphological 
and  physiological  importance.  Of  the  section  of  the  region  of  the  fore-brain 
little  need  be  said.  The  ectoderm  has  begun  to  thicken  somewhat.  The  walls  of 
the  fore-brain,  /.  b,  itself  have  not  begun  to  show  any  differentiation  into  layers. 
There  is  a  considerable  development  of  mesenchyma  between  the  brain  and  the 
superficial  ectoderm. 

Section  through  the  Venous  End  of  the  Heart  (Fig.  158). — We  have  now  passed 
in  our  series  beyond  the  level  of  the  head,  so  that  no  part  of  that  is  included  in 
the  section.  The  general  topography  of  the  part  is  similar  to  that  of  the  preced- 
ing section  (Fig.  157),  but  there  are  many  important  differences  of  detail.  We 
are  now  in  the  region  of  the  spinal  cord,  Sp.  c,  proper,  which  here  offers  to  us  its 
characteristic  early  embryonic  form.  It  is  oval  in  section,  its  walls  are  thickened 
on  each  side,  but  are  thinned  on  the  dorsal  side,  where  they  constitute  the  deck- 


EMBRYO   WITH  TWENTY-EIGHT  SEGMENTS. 


281 


plate,  and  on  the  ventral  side,  where  they  form  the  floor-plate;  the  cavity  is  nar- 
row and  slit-like.  The  notochord  lies  close  under  the  ventral  side  of  the 
medullary  tube  and  below  it  is  the  median  dorsal  aorta,  Ao,  a  single  and  very 
large  vessel,  which  is  formed  by  the  union  of  the  two  dorsal  aortae  shown  in  figure 
1 57,  Ao.  D.  Immediately  below  the  aorta  follows  the  pharynx,  Ph,  which  is  now 


Raph. 


FIG.  158. — SECTION  OF  CHICK  EMBRYO  WITH  ABOUT  TWENTY-EIGHT  SEGMENTS.  TRANSVERSE  SERIES  92, 

SECTION  144. 

Am,  Am',  Amnion.  Ao,  Aorta.  Au,  Cardiac  auricle.  Cho,  Chorion.  Cos,  Ccelorh.  D.  C,  Duct  of  Cuvier. 
EC,  Ectoderm.  Endo,  Endothelial  heart,  m.ht,  Muscular  heart,  msth,  Mesothelium.  My,  Primitive 
segment.  Ph,  Pharynx.  Raph,  Raphe  of  Amnion.  Soin,  Somatopleure.  Sp.c,  Spinal  cord.  Ven,  Ventricle 
of  heart.  X  5°  diams. 

more  rounded  in  form  and  does  not  extend  far  laterally.  Its  entodermal  lining 
is  moderately  thick,  but  it  is  somewhat  thinner  near  the  median  dorsal  line.  On 
either  side  of  the  pharynx  the  mesothelial  layer  is  very  thick  and  stands  out  con- 
spicuously, owing  to  its  dark  staining.  Above  the  pharynx  it  thins  out  and 


282  STUDY  OF  YOUNG  CHICK  EMBRYOS. 

passes  over  on  to  the  somatopleure,  Som,  and  so  on  to  the  amnion,  Am.  On  the 
ventral  side  of  the  pharynx  the  mesothelial  layer  passes  over  into  the  muscular 
wall  of  the  heart,  m.  hi.  The  heart  itself  is  very  large;  it  has  two  tubes,  the  en- 
dothelial,  endo,  and  the  muscular,  m.  ht,  which  are  very  distinct.  The  endothe- 
lial  cavity  is  very  large.  It  is  especially  expanded  immediately  underneath  the 
pharynx  to  form  the  auricular  end  of  the  heart,  which  receives  the  veins. 
Throughout  a  large  part  of  the  auricle  the  endothelium  is  closely  fitted  against 
the  muscular  wall.  Further  ventral  wards  the  double  heart-tube  bends  to  the 
right  of  the  embryo  to  form  the  ventricular  limb,  Ven,  in  which  the  endothelial 
cavity  is  also  enlarged.  The  heart  as  a  whole  occupies  about  one-half  the  area  of 
the  entire  section  of  the  embryo,  being  of  relatively  enormous  proportions.  The 
cardinal  veins,  D.  C,  have  moved  down  as  compared  with  the  previous  section, 
and  are  now  found  to  lie  in  the  somatopleure,  in  which  there  also  appear  several 
sections  of  smaller  blood  spaces  above  the  main  cardinal  vessel.  The  path  of  the 
cardinal  through  the  somatopleure  carries  it  toward  the  heart.  The  vertical 
part  of  the  vessel,  which  affects  a  union  with  the  heart,  is  known  as  the  duct  of 
Cuvier.  The  ducts  of  Cuvier  also  deliver  the  blood  from  the  posterior  cat dinals 
to  the  heart.  They  are  at  somewhat  different  levels  on  the  two  sides  of  the  em- 
bryo, that  on  the  right  side  being  lower  and  occupying  a  sort  of  prominence  on 
the  mesothelial  side  of  the  somatopleure.  If  the  cardinal  veins  are  traced  along 
through  successive  sections,  it  will  be  found  that  they  open  directly  into  the  auri- 
cles of  the  heart,  crossing  over  the  coelom,  Cos.  The  crossing  is  accomplished  by 
a  growth  of  the  somatopleure  which  unites  with  the  wall  of  the  heart.  The 
openings  of  these  veins  are  at  this  stage  morphologically  symmetrical  and  are 
entirely  distinct  from  the  openings  of  the  omphalo-mesaraic  veins,  which  enter 
the  heart  further  tailwards.  If  sections  in  the  series  between  the  present  one  and 
that  through  the  aortic  end  of  the  heart  (Fig.  157)  be  examined,  it  will  be  found 
that  the  heart  in  the  middle  part  of  its  course  is  entirely  detached  from  the 
pharynx,  so  that  the  heart-tube  is  suspended  by  its  two  ends  from  the  ventral 
side  of  the  pharynx.  By  the  crossing  of  the  cardinal  veins  the  portion  of  the 
coelom,  Cos,  on  either  side  of  the  pharynx  is  shut  off  from  the  portion  of  the  coelom 
around  the  heart.  At  the  raphe,  raph,  of  the  amnion  the  ectoderm  of  the  amniori 
joins  that  of  the  chorion,  Cho.  In  the  portion  of  the  somatopleure,  Am' ,  which 
runs  from  the  raphe  to  the  embryo  there  are  a  number  of  spaces  of  rounded  form 
which  appear  like  so  many  vesicles.  The  nature  of  these  vesicles  is  uncertain.* 


*  They  seem  to  be  bounded  on  one  side  by  ectoderm,  on  the  other  by  mesoderm  ;  but  as  to  this,  and  as 
to  the  significance  of  these  vesicles,  I  cannot  express  any  opinion.  The  separate  opening  of  the  ducts  of  Cuvier 
in  front  of,  and  independently  of,  the  omphalo-mesaraic  veins,  so  far  as  I  am  aware,  has  not  been"  hitherto 
recorded.  It  is  a  condition  of  morphological  importance. 


EMBRYOS  WITH  TWENTY-EIGHT  SEGMENTS.  283 

The  secondary  segments,  My,  are  very  characteristic,  and  should  be  studied 
with  a  higher  power.  The  segment  consists  of  an  outer  layer  and  an  inner  layer 
of  about  equal  thickness,  and  these  two  layers  pass  over  into  one  another  at  the- 
dorsal  and  ventral  edges  of  the  segment.  They  are  closely  pressed  against  one 
another,  so  that  there  is  no  space  between  them.  The  outer  layer  is  more  deeply 
stained  than  the  inner ;  its  nuclei  are  somewhat  less  distinct  and  are  rounded  in 
form.  Those  of  the  inner  layer  are  elongated  in  form,  as  may  be  easily  observed 
by  raising  and  lowering  the  focus.  The  outer  layer  is  quite  close  to  the  ectoderm, 
and  the  inner  layer  rests  against  the  large  mass  of  mesenchymal  tissue  which 
surrounds  the  spinal  cord,  notochord,  and  aorta. 

Section  through  the  Anlage  of  the  Liver  (Fig.  159). — In  this  section  the  gene- 
ral topography  is  similar  to  that  of  the  last,  so  that  we  need  describe  only  the  new 
structures  and  relations  which  appear.  A  little  piece  of  the  ventricular  limb  of 
the  heart  with  its  double  walls,  m.  hi.  endo,  still  appears.  The  section  is,  strictly 
speaking,  beyond  the  venous  end  of  the  heart  and  passes  through  the  sinus 
venosus,  Si.  V,  which  is  formed  by  the  union  of  the  omphalo-mesaraic  veins 
entering  the  body  of  the  embryo  from  the  splanchnopleure  of  the  yolk-sac,  or,  in 
other  words,  from  the  area  vasculosa.  In  the  splanchnopleure,  Spl,  there  is  a 
thickening,  x,  of  the  mesoderm  which  marks  the  crossing  of  the  veins  from  the 
yolk-sac  to  the  venous  end  of  the  heart.  The  entoderm  of  the  embryo  forms  a 
tube,  Ent,  which  is  greatly  elongated  in  its  dorso-ventral  diameter.  The  entoderm 
itself  is  quite  thick,  except  in  its  median  dorsal  portion.  From  the  ventral  side 
of  the  entodermal  canal  spring  two  small  pouches  or  diverticula,  the  anlages  of 
the  liver.  The  left  diverticulum  is  well  shown  in  the  figure ;  the  right  diverticulum 
appears  a  few  sections  further  on.  It  is  especially  important  to  note  that  the 
entodermal  epithelium  of  the  hepatic  diverticulum  comes  into  immediate  contact 
with  the  endothelium  of  the  blood  spaces.  During  the  later  development  this 
relation  is  preserved,  and  there  is  a  complicated  intercrescence  of  the  entodermal 
cells  constituting  the  liver  and  of  the  vascular  endothelium.*  The  intercrescence 
leads  to  the  formation  of  the  sinusoids,  which  are  highly  characteristic  of  the  liver 
and  which  give  rise  to  the  so-called  capillaries  of  the  hepatic  lobules  of  the  adult 
liver.  These  "capillaries"  are,  however,  always  true  sinusoids,  and  morpholog- 
ically not  capillaries  at  all.  Owing  to  the  junction  of  the  veins  and  liver,  a 
portion  of  the  body  cavity,  Coe' ' ,  at  the  side  of  the  pharynx  is  shut  off  from  direct 
connection  with  the  pericardia!  cavity.  The  ridge  of  tissue  dividing  the  two 
cavities  from  one  another  is  the  septum  transversum.  If  the  series  of  sections 
be  followed  through  tailward,  it  will  be  found  that  at  this  stage  further  back 

*  A  few  sections  anterior  to  this  the  beginning  of  the  intercrescence  is  observable. 


284 


STUDY  OF  YOUNG  CHICK  EMBRYOS. 


the  septum  transversum  is  formed  also  upon  the  right  side  of  the  body  of  the  em- 
bryo. The  mesothelium  between  the  upper  division  of  the  coelom,  Cos,  and  the 
sides  of  the  entodermal  canal  is  very  much  thickened  and  deeply  stained.  On 
either  side  of  the  very  large  median  aorta,  Ao,  and  just  above  the  coelom,  appear 


EC. 


Cho. 


msth. 


Ent. 


FIG.  159. — SECTION  OF  CHICK  EMBRYO  WITH  ABOUT  TWENTY-EIGHT  SEGMENTS.  TRANSVERSE  SERIES  92, 

SECTION  165. 

Am,  Amnion.  Ao,  Aorta,  card,  Cardinal  vein.  Cho,  Chorion.  Ca;  Cce' ,  Coelom.  EC,  Ectoderm,  endo, 
Endothelial  heart.  Ent,  Entoderm.  Li,  Liver,  mes,  Mesoderm.  m.ht,  Muscular  heart,  msth,  Meso- 
thelium. My,  Primitive  segment,  nch,  Notochord.  raph,  Raphe  of  amnion.  Si.  V,  Sinus  venosus  of 
heart.  Som,  Somatopleure.  Sp  c,  Spinal  cord.  Spl,  Splanchnopleure.  Ve,  Vein.  Y,  Accumulation  of 
mesodermic  tissue  about  the  omphalo-mesaraic  vein.  X  5°  diams. 

the  right  and  left  posterior  cardinal  veins,  card.  Concerning  the  foetal  envelopes 
little  need  be  said,  except  to  call  attention  to  the  large  raphe,  raph,  of  the  amnion, 
which  is  now  a  rather  conspicuous  ectodermal  thickening  and  seems  to  be  formed 


EMBRYO   WITH  TWENTY-EIGHT  SEGMENTS. 


285 


rather  at  the  expense  of  the  ectoderm  of  the  amnion  than  at  that  of  the  chorion. 
Such  an  ectodermal  raphe  is  very  characteristic  of  birds;  it  has  in  the  chick  a 
considerable  extent  and  therefore  appears  in  many  successive  sections  of  the 
series. 

Section  through  the  Omphalo-mesaraic  Veins  (Fig.   1 60). —This   section   is 
intermediate  in  structure  between  figure  159  and  figure  161,  here  described. 


Sp.c. 


Cho. 


Am. 


Sotn. 


Om  S. 


Om.D: 


FIG.  160. — SECTION  OF  CHICK  EMBRYO  WITH  ABOUT  TWENTY-EIGHT  SEGMENTS.  TRANSVERSE  SERIES  92, 

SECTION  179. 

Am,  Amnion.  Ao,  Aorta,  card,  Cardinal  vein.  Clio,  Chorion.  Cce,  Crelom.  Ent,  Entoderm.  /«,  Intestine. 
msth,  Mesothelium.  My,  Muscle  plate,  nch,  Notochord.  Om.D,  Right  omphalo-mesaraic  vein.  Om.S, 
Left  omphalo-mesaraic  vein.  Soi/i,  Somatopleure.  Sp.c,  Spinal  cord.  Spl,  Splanchnopleure.  X  5°  diams. 

We  are  now  beyond  the  region  of  the  heart  and  liver.  The  cavity  of  the  intes- 
tine is  open  on  the  ventral  side,  so  that  the  walls  of  the  intestine  pass  over 
directly  into  the  extra-embryonic  splanchnopleure,  Spl,  in  which  are  lodged 
the  very  wide  omphalo-mesaraic  veins,  Om.  D  and  Om.  S,  which  are  enter- 
ing the  body  of  the  embryo  to  run  forward  past  the  liver  anlage  (Fig.  159) 
to  join  the  posterior  or  venous  end  of  the  heart.  It  will  also  be  noticed 


286 


STUDY  OF  YOUNG  CHICK  EMBRYOS. 


that  the  amniotic  fold  does  not  join  its  fellow,  and  therefore  has  no  raphe.     In 
this  condition  the  amnion  is  said  to  be  "  open." 

Section  through  the  Anterior  Portion  of  the  Open  Intestine  (Fig.  161). — In  this 
section  the  intestinal  cavity,  In,  being  without  a  ventral  wall,  opens  directly  into 
the  general  eritodermal  cavity  under  the  germinal  area  and  above  the  yolk-mass. 
The  median  plane  of  the  embryo  is  still  inclined  to  the  left.  The  extra-embry- 
onic somatopleure,  Som,  rises  in  two  high  folds,  one  on  each  side  of  the  embryo; 
the  inner  portion  of  each  fold,  Am,  belongs  to  the  amnion,  the  outer  portion,  Cko, 
to  the  chorion.  The  splanchnopleure,  Spl,  passes  without  demarcation  into  the 
wall  of  the  intestinal  cavity,  In.  The  entoderm,  Ent,  of  the  extra-embryonic 


Som. 


EC.  msth.  card.   My,  tic  It.  Sp.c.  Cfi 
•        i 


b,w.     Am. 


Cho. 


In.     Ao.      Ve.  mes.      Enl.  Spl. 

FIG.  161. — SECTION  OF  A  CHICKEN  EMBRYO  WITH  ABOUT  TWENTY-EIGHT  SEGMENTS.     TRANSVERSE  SERIES 

92,  SECTION  220. 

Am,  Amnion.  Ao,  Aorta,  b.iv,  Body  wall,  card,  Right  posterior  cardinal  vein,  card.s,  Left  cardinal  vein. 
Cho,  Chorion.  EC,  Ectoderm.  Ent,  Entoderm.  In,  Intestine,  mes,  Splanchnic  mesoderm.  msth,  Meso- 
thelium.  My,  Myotome.  nch,  Notochord.  Som,  Somatopleure.  Sp.c,  Spinal  cord.  Spl,  Splanchnopleure. 
Ve,  Vein.  X  5°  diams. 

splanchnopleure  is  very  thin,  but  where  it  passes  into  the  embryonic  region  to- 
ward the  median  line,  it  thickens  a  little.  The  splanchnic  mesoderm  is  a  thin 
layer  of  mesothelium  which,  of  course,  bounds  the  coelom  everywhere  and  can 
be  followed  continuously  over  on  to  the  somatopleure.  The  splanchnic  mesen- 
chyma  is  loose  in  texture  and  surrounds  the  large  blood-vessels.  The  splanch- 
nic mesoderm  on  either  side  of  the  intestinal  groove  appears  quite  dark,  owing 
to  the  condensation  of  the  tissue.  Whether  this  condensation  is  developed  from 
the  mesothelium  or  from  the  mesenchyma  it  is  very  difficult  to  say.  The  soma- 
topleure, Som,  where  it  becomes  embryonic,  increases  greatly  in  thickness  and 
forms  an  arch,  b.  w,  which  is  the  beginning  of  the  formation  of  the  ventral  body- 


EMBRYO   WITH  TWENTY-EIGHT  SEGMENTS. 


287 


wall  of  the  chick.  The  form  of  the  arch  indicates  the  commencing  closure  of  the 
embryonic  somatopleure  on  the  ventral  side,  by  which  the  body  of  the  embryo 
will  ultimately  become  shut  off  from  the  underlying  layers  of  the  blastoderm.  In 
the  median  plane  of  the  embryo  we  find  the  spinal  cord,  cut  somewhat  obliquely, 
the  notochord,  nch,  and  the  very  large  section  of  the  aorta,  Ao.  The  great  trans- 
verse width  of  the  aorta  is  due  to  its  approaching  division  to  ward,  the  caudal  end 
of  the  body  to  form  the  two  branches  which  run  out  to  the  area  vasculosa  and 
are  known  as  the  omphalo-mesaraic  or  vitelline  arteries.  Before  they  leave  the 
body  of  the  embryo  each  of  these  arteries  gives  off  a  branch  which  continues  in 
the  body  of  the  embryo  not  far  from  the  notochord  and  close  to  the  entoderm. 
These  branches  subsequently  become  the  allantoic  arteries.  On  either  side  of 
the  spinal  cord  lie  the  secondary  segments,  My.  A  short  distance  from  the 


SOHI. 


IV.  D.  .  Se 


Ahs. 


Cce.  Spl.  Ve.         nch.  Ent.  Spl.         mes' '. 

FIG.  162. — SECTION  OF  A  CHICKEN  EMBRYO  WITH  ABOUT  TWENTY-EIGHT  SEGMENTS.     TRANSVERSE  SERIES 

92,  SECTION  356. 

Cce,  Ccelom.  EC,  Ectoderm.  Ent,  Entoderm.  Mes,  Somatic  mesoderm.  mes',  Splanchnic  mesoderm.  N, 
Xephrotome.  nch,  Notochord.  Stg,  Segment.  Som,  Somatopleure.  Sp.c,  Spinal  cord.  Spl,  Splanchno- 
pleure.  Ve,  Blood-vessel.  IV.  D,  Wolffian  duct.  X  5°  diams. 

aorta  on  either  side  appear  sections  of  two  rather  small  blood-vessels,  the  cardinal 
veins,  card.  Between  the  vein  on  each  side  and  the  aorta  there  is  a  little  accu- 
mulation of  denser  tissue.  "  If  a  series  of  sections  is  followed  through,  the  Wolff- 
ian duct  may  be  traced  into  this  condensed  tissue,  and  when  the  duct  is  differen- 
tiated, it  will  take  the  place  of  this  tissue  between  the  aorta  and  the  vein. 

Section  through  the  Middle  Portion  of  the  Open  Intestine  (Fig.  162) . — Compari- 
son of  this  section  with  the  preceding  is  instructive  as  an  illustration  of  the  fact 
that  the  differentiation  of  structures  is  found  less  advanced  as  we  proceed  toward 
the  caudal  end  of  the  embryo.  In  the  present  section  the  amniotic  folds  can 
hardly  be  said  to  have  appeared  at  all,  although  the  coelom,  Coe,  is  very  wide  in- 
deed, and  there  is  little  differentiation  in  either  the  somatopleure,  Som,  or  splanch- 


288 


STUDY  OF  YOUNG  CHICK  EMBRYOS. 


nopleure,  Spl,  between  the  embryonic  and  extra-embryonic  regions.  The  ento- 
derm  is  a  little  thicker  in  the  embryo  than  in  the  extra-embryonic  territory.  A 
similar  difference  may  be  observed  in  the  ectoderm.  The  embryonic  mesoderm  in 
both  somatopleure  and  splanchnopleure  is  considerably  more  developed  and  much 
denser  than  in  the  extra-embryonic  parts.  The  axial  structures  of  the  embryo — 
namely,  the  spinal  cord,  Sp.  c,  and  notochord,  nch — are  about  the  same  as  further 
forward,  but  the  mesoderm  is  much  less  advanced  than  further  headwards,  as  is 
evidenced  by  the  small  amount  of  mesenchyma  above  the  axial  structures  and 
by  the  slight  differentiation  of  the  mesothelium.  The  condition  of  the  segments 
and  their  relations  to  the  somatic  and  splanchnic  mesoderm  are  closely  similar  to 
those  represented  in  figure  32.  Each  segment  consists  of  a  larger  part,  Seg,  of 
rounded  outline,  close  to  the  medullary  tube,  and  of  a  narrower  part,  the  nephro- 


Som.      Cce. 


Co:.' 


nch. 


mes.      In.      /:';//. 


FIG.   163.— SECTION  OF  A  CHICKEN   EMBRYO  WITH   TWENTY-EIGHT  SEGMENTS.     TRANSVERSE   SERIES  92, 

SECTION  419. 
Cce,  Coelom.      Cce' ',  Diverticulum  of  the  ccelom.     Ent,  Entoderm.     In,  Intestinal  cavity,    mes,  Mesoderm.    ;/r//, 

Notochord.     Som,  Somatopleure.     Sp.c,  Spinal  cord.    Spl,  Splanchnopleure.     S.z,  Segmental  zone.     X  5° 

diams. 

tome,  N,  which  connects  the  inner  portion  of  the  segment  with  the  lateral  meso- 
derm. The  secondary  segment  consists  of  a  distinctly  marked  wall  which 
extends  around  underneath  the  ectoderm  and  against  the  side  of  the  medullary 
tube,  and  of  a  thick  inferior  wall  which  fills  up  also  the  center  of  the  segment. 
Between  the  nephrotome  and  the  entoderm  are  small  blood-vessels,  Ve. 

Section  through  the  Posterior  Portion  of  the  Open  Intestine  (Fig.  163). — This 
section  is  similar  to  the  last,  but  we  may  note  especially  the  following  differ- 
ences: The  spinal  cord,  Sp.  c,  shows  a  comparatively  large  .cavity,  which 'is 
widest  on  the  dorsal  side,  so  as  to  be  somewhat  triangular  in  section.  In  place  of 
the  segments  we  have  only  the  mass  of  cells,  S.  z,  which  constitutes  the  segmental 
zone,  out  of  which  later  segments  will  be  differentiated.  The  segmental  zone  is 
of  a  rather  loose  texture  and  merges  without  boundary  into  the  somewhat  denser 


EMBRYO   WITH  TWENTY-EIGHT  SEGMENTS.  289 

mesenchyma  of  the  somatopleure  and  splanchnopleure  of  the  embryo.  The 
dense  tissue  of  the  somatopleure  extends  much  further  laterally  than  the  corre- 
sponding tissue  in  the  splanchnopleure.  The  notochord  is  very  large  and  fills 
out  the  entire  space  between  the  ventral  boundary  of  the  spinal  cord  and  the 
entoderm,  and  though  the  mesoderm  comes  in  contact  with  the  notochord,  it  does 
not  surround  it,  the  relations  here  representing  an  earlier  stage  of  development 
than  any  which  we  find  further  head  wards.  The  entoderm,  Ent,  of  the  embryonic 
region  is  considerably  thickened  and  forms  an  intestinal  channel,  In,  of  very  char- 
acteristic form;  for  the  top  of  this  channel  is  nearly  horizontal,  while  the  sides  are 
vertical  and  form  a  distinct  angle  with  the  top.  In  the  midst  of  the  mesoderm, 
on  either  side  of  the  intestine,  there  is  a  small  cavity,  Cce',  which  in  two  or  three 
sections  further  forward  is  found  to  unite  with  the  general  cavity  of  the  coelom. 
The  morphological  meaning  of  this  special  pocket  of  the  body-cavity  is  unknown. 

Soin.       EC.  Cx.  cau.i..    Sp.c.     nek.    S.z.  Ales. 


Ent.  All. 


FIG.  164. — CHICK  EMBRYO  WITH  ABOUT  TWENTY-EIGHT  SEGMENTS.     TRANSVERSE  SERIES  92,  SECTION  424. 

All,  Allantois.  cau.i,  Caudal  intestine.  Cce,  Coelom.  EC,  Ectoderm.  Ent,  Entoderm.  Mes,  Mesoderm. 
mesf,  Splanchnic  leaf  of  mesoderm.  nek,  Notochord.  Som,  Somatopleure.  Sp.f,  Spinal  cord.  Spl, 
Splanchnopleure.  S.z,  Segmental  zone  of  mesoderm.  Ve,  Blood-vessel.  X  5°  diams. 

From- this  point  onward  in  the  series  changes  in  the  appearance  of  the  sec- 
tions take  place  very  rapidly.  The  two  sections  next  to  be  described  are  quite 
close  in  the  series  to  the  present  one. 

Section  through  the  Caudal  Intestine  (Fig.  164). — In  this  section  we  encounter 
the  singular  fusion  of  the  germ-layers  which  is  characteristic  of  the  caudal  extrem- 
ity of  all  vertebrate  embryos  during  early  stages.  In  the  median  line  we  see  three 
distinct  cavities.  The  dorsal  of  these  may  be  readily  identified  as  the  continua- 
tion of  the  cavity  of  the  spinal  cord.  The  middle  and  ventral  cavities  are  ento- 
dermal; the  upper  of  the  two  entodermal  cavities,  can.  i,  represents  a  prolonga- 
tion of  the  entodermal  cavity  into,  the  developing  tail  of  the  embryo  (compare 
Fig.  43).  The  lower  cavity  is  the  anlage  of  the  allantois,  All,  which  is  destined 
to  grow  out  during  the  next  few  days  into  a  relatively  large  round  vesicle.  The 
tissue  on  the  ventral  side  of  the  spinal  cord,  Sp.  c,  is  connected  by  a  band  of  cells 
19 


290 


STUDY  OF  YOUNG  CHICK  EMBRYOS. 


with  the  wall  of  the  caudal  intestine,  can.  i.  If  the  sections  just  in  front  are 
studied  carefully,  it  can  be  easily  observed  that  the  notochord  also  passes 
over  without  boundary  into  the  same  band  of  cells,  which  is  a  mass  repre- 
senting the  fusion  of  the  walls  of  the  medullary  canal  of  the  intestine  and  of  the 
tissue  of  the  notochord.  In  this  fused  tissue  we  can,  with  our  present  means, 
detect  no  signs  of  the  coming  differentiation.  Just  as  the  walls  of  the  caudal 
intestine  are  fused  with  the  tissues  on  the  dorsal  side,  so  also  are  they  fused  on  the 
ventral  side  with  the  tissue  of  the  allantois.  If  we  follow  the  tissues  laterally, 
we  see  that  they  merge  into  the  mesoderm  proper.  From  the  mesoderm  there 
has  been  a  distinct  upgrowth  of  tissue  of  rather  loose  texture  on  either  side  of  the 
medullary  canal  to  form  the  segmental  zone,  S.  z. 

Section  through  the  Allantois  behind  the  Intestine  (Fig.  165). — This  section  is 
only  three  in  the  series  beyond  that  last  described,  yet  it  is  posterior  to  the  caudal 


EC.   '  Mes.  Sp.c.     nch.     S  z. 


Som. 


mes.f       Ent.  All.  Ve.     msth.       Spl. 

FIG.  165. — CHICK  EMBRYO  WITH  ABOUT  TWENTY-EIGHT  SEGMENTS.  TRANSVERSE  SERIES  92,  SECTION  427. 
All;  Allantoisl  Cce,  Coelom.  EC,  Ectoderm.  Ent,  Entoderm.  Mes,  Mesoderm.  mes',  Splanchnic  mesoderm. 

msth,  Mesottielium.     nch,  Notochord.     Som,  Somatopleure.    Sp.c,  Spinal  cord.     Spl,  Splanchnopleure.   S.z, 

cjegmental  zone  of  mesoderm.      Ve,  Blood-vessel.     X  5°  diafns. 

intestine  and  shows,  therefore,  more  completely  the  fusion  of  the  structures  in 
the  axial  region.  Except  for  the  absence  of  the  caudal  intestine,  the  description 
of  the  last  section  might  apply  also  to  this.  The  shape  of  the  spinal  cord,  Sp.  c, 
is  somewhat  different,  and  its  merging  on  the  ventral  side  with  the  underlying 
tissues  is  more  marked.  The  cavity  of  the  allantois  is  smaller  and  almost  slit- 
like.  The  other  differences  do  not  call  for  special  description. 

Horizontal  Section. — The  student  will  find  it  profitable  to  make  a  series  of 
sections>in  the  horizontal  plane,  trying  to  cut  them  as  nearly  as  possible  parallel 
with  the  median  plane  of  the  fore-brain  and  mid-brain. 

The  accompanying  figure  166  is  from  a  section  of  such  a  series.  It  shows 
very  clearly  the  general  form  of  the  embryo,  the  curvature  of  the  neck,  the  sharp 
angle  of  the  head-bend,  and  the  almost  straight  body.  In  the  section  repre- 
sented the  long  stretch  of  the  cavity  of  the  fourth  ventricle  or  hind-brain,  Ven.  IV, 


M.b. 


Yen.  IV. 


Ao. 


Am". 


Scg.z. 


Sp.c. 


FIG.  166. — HORIZONTAL  SECTION  OF  A  CHICK.  EMBRYO  WITH  ABOUT  TWENTY-EIGHT  SEGMENTS. 
Am,  Am',  Am",  Amnion.  Ao,  Aorta.  C.ao,  Cardiac  aorta.  Cce,  Coelom.  D.Ao,  Dorsal  aorta.  Dien,  Dien- 
cephalon.  Endo,  Endothelial  heart.  H,  Cerebral  hemisphere.  M.b,  Mid-brain.  Mdb,  Mandible.  Jl/d, 
Medullary  tube,  m.ht,  Muscular  heart,  nch,  nch' ,  nth",  Notochord.  Op,  Optic  vesicle.  Ph,  Pharynx. 
Seg,  Segment.  Seg.z,  Segmental  zone  of  mesoderm.  Som,  Somatopleure.  Sp.c,  Spinal  cord.  J'en,  Ven- 
tricle. Ven.  IV,  Fourth  ventricle  or  cavity  of  the  hind-brain.  X  3°  diams. 

291 


292  STUDY  OF  YOUNG  CHICK  EMBRYOS. 

is  well  shown,  and  it  can  be  readily  seen  that  the  hind-brain  is  nearly  equal  in 
length  to  the  mid-  and  fore-brains  combined.  In  the  floor  of  the  hind-brain  ap- 
pears a  series  of  curved  notches  corresponding  to  the  neuromeres.  Only  a  shaving 
from  the  side  of  the  mid-brain,  M.  b,  and  two  similar  shavings  from  the  two  parts 
of  the  fore-brain,  the  diencephalon,  Dien,  and  the  cerebral  hemispheres,  H,  appear 
in  the  section.  The  optic  nerve  is  cut  across  and  appears  as  a  hollow  tube. 
Underneath  the  hind -brain  a  piece  of  the  pharynx,  Ph,  is  cut,  and  below  the 
pharynx  is  the  large  projecting  heart,  which  is  very  clearly  shown  to  consist  of  an 
inner  or  endothelial  tube,  Endo,  and  an  outer  mesothelial  tube,  m.ht,  the  anlage 
of  the  muscular  portion  of  the  heart.  The  endothelial  tube  is  cut  twice,  the 
upper  portion,  Ao,  is  the  aortic  trunk,  the  lower  portion,  Ven,  corresponding  to 
the  ventricle.  The  heart  is,  as  it  were"  suspended  from  the  lower  wall  of  the 
pnarynx.  The  entoderm  of  the  pharynx  is  very  thin  on  the  dorsal  side,  and 
thicker  on  the  ventral  side.  Between  the  head  and  the  pharynx  one  can  see  the 
projecting  mandibular  process,  Mdb.  The  small  space  to  the  right  of  this  process 
in  the  figure,  between  it  and  the  head,  corresponds  to  the  cavity  of  the  mouth. 
Close  to  the  mandibular  process,  on  the  side  toward  the  heart,  springs  the  amnion 
of  the  embryo,  Am,  which  passes  close  around  the  head  of  the  embryo  lying  very 
near  it,  and  can  be  followed  down  to  where  it  rejoins  the  posterior  end  of  the 
embryo,  on  the  left-hand  side  of  the  figure.  Underneath  the  posterior  part  of  the 
hind-brain  can  be  seen  a  small  piece  of  the  notochord,  nch.  The  notochord 
appears  twice  more  in  the  section,  nch'  and  nch" ,  in  the  dorsal  region  of  the  em- 
bryo. From  the  end  of  the  hind-brain  the  cervical  region  curves  to  the  right. 
In  it  there  is  a  large  cavity,  D.  Ao,  the  dorsal  aorta.  To  the  left  of  the  dorsal 
aorta  we  begin  to  get  the  primitive  segments,  which  are  very  distinctly  marked. 
They  become  gradually  wider  and  wider  as  we  proceed  toward  the  caudal  end 
of  the  embryo.  There  also  they  are  less  advanced  in  their  development.  A 
small  bit  of  the  spinal  cord  appears  in  section,  Md.  From  the  extreme  inferior 
end  of  the  section  a  prolongation  of  the  splanchaopleure  can  be  seen  which  also 
leads  off  into  the  formation  of  the  amnion,  Am".  There  appears  again  a  piece, 
Sp.c,  of  the  spinal  cord  and  a  fragment  of  the  notochord,  and  on  either  side  of  this 
a  segmental  zone,  Seg.  z,  of  the  mesoderm.  On  the  right  there  shows  a  small 
portion  of  the  body-cavity,  Cos,  distinctly  bounded  on  both  sides.  Its  exterior 
boundary  is  a  piece  of  the  true  body- wall,  Som,  of  the  embryo,  and  close  by  it  is 
another  portion  of  the  amnion,  Am'.  How  this  is  possible  may  be  readily  under- 
stood by  comparison  of  this  figure  with  figure  161,  which  represents  a  transverse 
section  of  a  similar  embryo  in  this  region. 


HISTOLOGICAL  DIFFERENTIATION.  293 

Histological  Differentiation  of  the  Chick  Embryo  with  Three  Gill  Clefts. 

It  is  important  that  the  student  make  a  thorough  examination  and  study 
with  a  high  power  of  all  the  cells  and  tissues  of  the  embryo  at  this  stage  so  as  to 
familiarize  himself  with  the  embryonic  characteristics  of  the  germ-layers.  The 
cellular  homogeneity  of  the  embryo  is  strikingly  evidenced  by  the  nuclei,  which 
in  all  parts  of  the  embryo  are  very  similar  in  size,  shape,  and  structure.  They 
are  all  rounded  in  form,  varying  between  spherical  and  slightly  oval  outlines, 
which  are  seldom  quite  regular.  The  outline  of  the  nucleus  is  always  well 
marked,  there  being  a  superficial  layer  of  nuclear  substance,  which  gives  a  darker 
appearance  to  the  edge  of  the  nucleus.  In  the  interior  there  is  a  single  or  some- 
times two,  very  rarely  three,  nucleoli,  which  are  quite  large  and  stain  deeply. 
The  strands  of  substance  between  the  nucleolus  and  the  outer  part  of  the  nucleus 
are  very  slight,  and  the  space  around  the  nucleolus,  therefore,  appears  light. 
The  protoplasm  of  the  cells  is  never  large  in  amount,  so  that  the  cell-body  about 
each  nucleus  is  not  conspicuous,  except  in  the  case  of  the  blood-corpuscles,  which 
are,  in  this  respect,  somewhat  more  advanced  than  the  other  cells  of  the  embryo. 

The  ectoderm  offers  chiefly  variations  in  its  thickness,  being  very  much  at- 
tenuated in  some  parts,  as,  for  instance,  in  the  posterior  portion  of  the  head,  where 
the  outer  ectoderm  overlies  the  hind-brain.  Most  of  the  epidermal  parts  have 
begun  to  increase  in  thickness,  and  contain  nuclei  in  two  or  even  three  layers. 
There  are  several  special  thickenings  of  the  epidermal  layer,  for  which  the  name 
of  plakodes  has  been  proposed.  At  the  present  stage  three  pairs  of  plakodes  are 
seen.  The  first  is  the  pair  of  areas  which  are  to  be  invaginated  to  form  the 
olfactory  pits;  the  second  is  the  pair  which  are  already  invaginated  to  form 
the  anlages  of  the  lenses  of  the  eyes,  and  the  third  pair  are  also  invaginated  to 
form  the  otocysts.  The  portion  of  the  ectoderm  which  forms  the  medullary  tube 
is  also  very  much  thickened,  except,  of  course,  so  far  as  the  floor-plate  and  deck- 
plate  have  been  differentiated.  In  both  the  plakodes  and  in  the  thickened  por- 
tions of  the  medullary  wall  the  nuclei  occupy  nearly  the  whole  thickness  of  the 
layer,  being  themselves  several  layers  deep.  They  are,  however,  partially 
absent  from  that  portion  of  the  ectoderm  which  is  near  the  original  external  or 
free  surface.  Close  to  this  surface  there  are,  however,  a  certain  number  of 
nuclei,  the  large  majority  of  which  are  in  various  phases  of  division,  as  shown  by 
the  numerous  mitotic  figures.  No  mitoses  appear,  except  in  the  superficial  por- 
tion of  the  layer.  Over  the  greater  part  of  the  amnion  the  ectoderm  is  so  very 
thin  as  to  resemble  almost  an  adult  endothelium,  but  over  the  chorion  or  serous 
membrane  it  is  a  little  thicker. 

The  entoderm  appears  in  three  distinct  forms:  first,  the  large,  long, 
columnar  cells  of  the  area  opaca ;  second,  the  very  thin  cells  of  the  area  pellucida ; 


294  STUDY  OF  YOUNG  CHICK  EMBRYOS. 

and,  third,  the  somewhat  thicker  cell-layer  in  the  embryo  proper.  For  an  ac- 
count of  the  cells  of  the  area  opaca  and  area  pellucida  (see  page  86'.  The  ento- 
derm  in  the  embryo  presents  considerable  variations  in  thickness  which  have 
been  pointed  out  in  the  descriptions  of  the  sections.  Where  it  Is  thick  enough 
to  permit  it,  the  nuclei  are  disposed  in  several  layers,  and  in /-such  places  we  find 
that  the  nuclear  divisions  take  place  only  in  the  superficial  portion  of  the  ento- 
derm,  the  phenomenon  here  being  similar  to  that  which  we  have  already  noted  in 
the  ectoderm.  The  notochord  has  a  sharply  defined  outline,  as  if  bounded  by 
a  distinct  membrane.  It  contains  nuclei  which  are  quite  closely  placed,  but  it 
does  not  show,  at  least  in  ordinary  preparations,  any  recognizable  division  into 
separate  cells. 

The  mesoderm  offers  several  varieties,  not  so  much  in  the  character  of  the 
single  cells  as  in  their  methods  of  grouping.  We  notice,  first,  that  there  are 
parts  of  the  mesoderm  which  are  quite  thick,  and  in  which  we  cannot  perceive 
any  division  into  mesothelium  and  mesenchyma.  Such  a  thick  layer  of  mesoderm 
may  be  observed  at  either  side  of  the  pharynx  (-Figs.  158,  Ph,  and  159),  or,  again, 
toward  the  caudal  end  of  the  embryo  in  both  the  somatopleure  and  splanchno- 
pleure,  occupying  a  larger  territory  in  the  former  than  in  the  latter  (Fig.  163). 
But  for  the  most  part  the  mesoderm  has  progressed  beyond  this  stage  and  shows 
clearly  the  differentiation  of  a  thin  mesothelial  layer  lining  the  ccelom  and  the 
scattered  mesenchymal  cells.  The  mesothelium  is  quite  thin  in  some  parts, 
almost  or  quite  as  thin  as  adult  endothelium.  The  mesenchyma  consists  of  cells 
with  small  protoplasmic  bodies  connected  together  by  fine  threads  of  protoplasm 
and  with  a  transparent  homogeneous  matrix  between  the  cells.  It  varies  greatly 
in  appearance  according  as  the  cells  are  more  or  less  closely  crowded  together,  or 
widely  separated  from  one  another.  These  differences  we  designate  as  varying 
degrees  of  condensation  in  the  mesenchyma.  The  variations  occur  in  a  per- 
fectly definite  and  constant  manner,  though  we  are  far  from  understanding  yet 
either  the  cause  or  the  morphological  significance  of  these  variations.  The  sec- 
ondary segments  vary  greatly  in  structure,  because  they  are  in  unlike  stages  of 
differentiation,  those  toward  the  tail  being  least,  and  those  in  the  cervical  region 
most  advanced.  We  can,  therefore,  in  a  single  embryo  observe  several  phases 
of  the  breaking-up  of  the  inner  wall  of  the  segment  to  form  mesenchyma  about 
the  medullary  tube  and  notochord.  The  transformation  is  accomplished  by  a 
spreading  out  and  moving  asunder  of  the  cells,  and  we  can  also  trace  a  gradual 
differentiation  of  the  muscle  plate,  out  of  the  inner  portion  of  the  segment.  The 
external  layer,  or  so-called  cutis  plate,  offers  an  apparently  more  or  less  epithe- 
lioid  structure  in  all  of  the  segments.  The  Wolffian  duct  is  differentiated  only 
through  a  part  of  the  embryo.  It  is  a  small  cord  of  cells  that  has  as  yet  no  cen- 


EMBRYO   WITH  SEVEN  SEGMENTS.  295 

tral  cavity.  The  blood-vessels  are  formed  solely  by  the  endothelium  (angio- 
blast).  There  is  nowhere  any  condensation  of  the  mesenchyma  about  the  blood- 
vessels as  yet.  There  are  no  capillaries  whatever  in  the  embryo.  One  of  the 
most  important  vascular  modifications  has,  however,  been  initiated  in  the  anlage 
of  the  liver,  where  we  find  the  vascular  endothelium  coming  into  close  contact 
with  the  entodermal  cells  of  the  liver,  preparatory  to  the  later  complete  differen- 
tiation of  the  hepatic  sinusoids.  The  blood-corpuscles  are  round  in  form  with 
fairly  distinct  outlines.  Their  protoplasmic  bodies  are  much  larger  than  those  of 
any  other  cells  of  the  embryo  at  this  stage,  but  their  nuclei  resemble  in  size  and 
structure  those  of  other  tissues. 

Embryo  Chick  with  Seven  Segments.     (About  twenty-seven  hours'  incubation.) 
The  following  description  is  almost  equally  applicable  to  embryos  with  five 
or  nine  segments. 

Examination  in  the  Fresh  State. — The  embryo  when  first  removed  from  the 
yolk  should  be  placed  in  a  staining-dish  with  a  small  quantity  of  normal  salt 
solution  and  examined  with  a  low  power  of  the  microscope  as  a  transparent 
object.  The  specimen  as  a  whole  has  a  grayish  or  brownish-gray  tint.  Most 
of  the  germinal  area  is  dark,  the  transmission  of  light  being  stopped  by  the 
numerous  yolk-grains  contained  in  the  entodermal  cells  (compare  page  87).  In 
the  center  of  the  germinal  area  the  translucent  area  pellucida  is  very  conspicu- 
ous, and  has  an  edge  which  is  quite  sharply  defined,  more  so  than  after  the  speci- 
men has  been  preserved.  It  is  shaped  somewhat  like  an  elongated  pear.  In  the 
axial-  portion  of  the  area  the  embryonic  structures  are  partially  differentiated 
(Fig.  167).  It  should  be  noted  that  this  figure  is  taken  from  a  hardened,  not  a 
fresh  specimen.  The  head  of  the  embryo  is  protuberant  and  is  of  a  bluntly 
rounded  form.  It  projects  freely  above  the  surface  of  the  germinal  area.  Under- 
neath the  projecting  head  is  a  very  clear  area  in  which  there  is  no  mesoderm  what- 
ever. This  is  the  pro-amnion,  pro.  am.  On  either  side  of  the  head  are  two  char- 
acteristic spaces,  a.  c.  i>,  the  amnio-cardiac  vesicles.  The  surface  of  the  germinal 
area  rises  somewhat  dome-like  over  each  vesicle.  This  is  due  to  the  fact  that  in 
this  region  the  ccelom  is  already  very  large  and  the  splanchnopleure  or  upper 
leaf  of  the  germinal  area  is  arched  on  either  side  of  the  embryo.  The  relations 
may  be  more  clearly  understood  from  cross-sections  (Fig.  170).  The  posterior 
limit  of  the  head  is  marked  by  an  arching  line,  the  concavity  of  which  faces  the 
caudal  end  of  the  embryo.  This  line,  fov,  marks  the  position  of  the  fo-uea  car- 
diaca.  On  the  sides  of  the  fovea,  running  forward  toward  the  median  line  of  the 
embryo,  one  can  distinguish  two  darker  bands  which  represent  the  beginning  of 
the  formation  of  blood-vessels  growing  in  from  the  extra-embryonic  region  to  join 


296 


STUDY  OF  YOUNG  CHICK  EMBRYOS. 


,-pro.am 


fov 


in  the  median  line  of  the  embryo  and  participate  in  the  formation  of  "the  heart  on 
the  under  side  of  the  head.  In  order  to  see  the  fovea  clearly,  the  focus  of  the 
microscope  must  be  lowered.  The  medullary  groove  is  partly  converted  into  the 
medullary  canal,  for  at  this  stage  it  is  closed  from  the  anterior  limit  of  the  head 
to  a  variable  point  in  the  segmented  region  of  the  embryo.  Specimens,  however, 
vary  extremely  as  to  the  extent  of  the  closure.  The  line  of  closure  can  be 
readily  seen.  It  is  somewhat  wavy  and  irregular  in  its  course,  and  the  closure 
itself  is  somewhat  irregular,  so  that  we  may  find  one  or  several  points  where  the 

closure  is  not  yet  completed,  although  it 
is  complete  behind  and  in  front  of  these 
points.  At  the  anterior  extremity  of  the 
head  closure  is  always  .incomplete,  there 
being  an  opening  there  which  persists  for 
some  time  and  is  known  as  the  anterior 
neuropore.  Above  the  primitive  seg- 
ments, where  it  is  not  closed,  the  medul- 
lary groove  has  its  edges  close  together. 
But  a  short  distance  behind  the  last 
segment  the  groove  widens  .abruptly  and 
fades  out  gradually.  This  widening  is 
termed  the  rhomboidal  sinus.  By  proper 
adjustment  of  the  focus  the  notochord 
may  be  distinguished  underneath  this 
sinus.  Just  at  the  posterior  limit  of  the 
sinus  the  primitive  groove,  pr,  begins  and 
extends  backward,  often  bending  to  one 
side  or  the  other,  usually  to  the  left.  The 
groove  is  shallow  in  front,  deeper  behind, 
and  ends  quite  abruptly.  The  primitive 
segments  appear  as  square  darkish  blocks 
of  tissue  symmetrically  placed  on  either 
side  of  the  medullary  canal.  The  first 
pair  of  blocks  lies  a  short  distance  behind 

the  fovea  and  the  last  pair  a  short  distance  in  front  of  the  rhomboidal  sinus. 
When  new  segments  are  added,  they  are  about  the  same  size  as  those  previously 
formed.  The  growth  of  the  embryo  in  length  during  these  stages  depends 
rather  upon  the  multiplication  of  the  segments  than  upon  the  growth  of  the 
single  segments.  The  region  about  the  primitive  streak  is  quite  dark,  owing 
to  the  accumulation  of  cells,  which  belong  chiefly  to  the  mesoderm.  This  dark 


pr 


A.P 


FIG.  167. — CHICKEN  EMBRYO,  AFTER  TWENTY- 
SEVEN  HOURS'  INCUBATION,  WITH  EIGHT 
PRIMITIVE  SEGMENTS. 

Fovea  cardiaca.  pro. am,  Pro-amnion.  a.c.v, 
Amnio-cardiac  vesicle,  sf,  Sinus  terminalis. 
pr,  Primitive  groove.  Ao,  Area  opaca.  A.p, 
Area  pellucida. — (After  Duvctl.} 


f<n 


EMBRYO   WITH  SEVEN  SEGMENTS.  297 

appearance  extends  forward  and  merges  into  the  so-called  segmental  zone,  out 
of  which  the  segments  are  differentiated.  More  careful  examination  of  the 
area  opaca  shows  that  it  already  possesses  a  well-defined  area  vasculosa,  Ao,  in 
which  (with  some  difficulty  in  the  fresh  specimen)  traces  of  the  formation  of 
blood-vessels  and  blood-islands  can  be  made  out.  They  can  be  somewhat  better 
distinguished  if  the  fresh  specimen  be  examined  not  by  transmitted,  but  by  re- 
flected light.  In  the  fresh  specimen  it  is  very  difficult  to  make  out  the  slia-pe  of 
the  medullary  canal  in  the  region  of  the  head. 

Examination  of  the  Specimen  after  Hardening. — The  specimen,  after  it  has 
been  hardened,  should  be  examined  under  the  microscope  in  water  or  alcohol ;  and, 
again,  after  it  has  been  stained  it  should  be  cleared  in  oil  and  further  examined. 
This  will  enable  the  student  to  make  out  the  blood-islands  and  something  of  the 
blood-vessels  in  the  area  vasculosa,  and  also  the  shape  of  the  brain  which  (Fig. 
167)  has  expanded  widely  just  behind  the  neuropore;  the  lateral  expansions  are 
the  anlages  of  the  optic  vesicles  (Fig.  1 56) .  The  remainder  of  the  brain  extends 
from  the  optic  enlargement  to  a  point  a  little  behind  the  fovea,  JOD.  It  is  much 
wider  than  the  remaining  portion  of  the  medullary  canal;  it  tapers  from  the 
optic  vesicle  and  extends  backward.  One  cannot  yet  distinguish  in  it  positively 
any  subdivision  into  mid-brain  and  hind-brain.  On  the  contrary,  its  walls  are 
often  somewhat  irregularly  sinuous  and  vary  considerably  from  specimen  to 
specimen. 

Comparison  -with  a  Rabbit  Embryo. — In  the  ovum  of  the  mammalia  the 
ectoderm  presents  a  modification  known  as  the  trophoblast.  In  the  rabbit 
this  trophoblast  is  developed  over  a  limited  region  which  is  called  the  placental 
area  (Fig.  168,  a. pi],  by  which  the  embryo  is  attached  to  the  wall  of  the  uterus. 
When  the  embryo  figured  was  removed,  a  portion  of  the  placental  area  re- 
mained attached  to  the  uterus,  hence  the  defect  shown  in  the  specimen.  The 
vascular  area  is  nearly  circular;  its  boundary  is  marked  by  a  well-defined 
terminal  vessel,  v.t.  The  nearly  straight  embryo  lies  in  the  center  and  exhibits 
plainly  the  medullary  canal  and  primitive  segments.  The  optic  evaginations 
are  already  present.  The  head  is  free;  on  its  under  side  the  heart  is  forming, 
and  beneath  it  is  a  relatively  large  and  conspicuous  pro-amnion,  pr.  a.  Blood- 
vessels are  present  over  the  area  vasculosa,  but  not  yet  in  the  embryo.  It 
will  be  seen,  therefore,  that  though  the  proportions  differ  greatly  from  those 
in  the  chick,  the  fundamental  relations  in  the  rabbit  are  the  same  as  in  the 
bird. 

Longitudinal  Section  of  a  Chick. — In  order  to  facilitate  the  study  of  the 
transverse  sections  of  this  stage,  figure  169  is  inserted,  which  is  a  nearly  median 
longitudinal  section.  In  consequence  of  the  head  end,  H,  having  grown  for- 


298 


STUDY  OF  YOUNG  CHICK  EMBRYOS. 


ward  about  the  pro-amnion,  pro.a,  it  has  become  free  on  all  sides,  and  at  the 
same  time  the  entodermal  cavity  has  been  carried  forward  with  the  head, 
making  the  so-called  fore-gut  of  English  authors.  This  fore-gut  is  the  anlage 


FIG.  1 68. — EMBRYONIC  AREA  OF  A  RABBIT  OF  (?  NINE)  DAYS,  WITH  THE  PLACENTAL  AREA  PARTLY  TORN  OFF. 
pr.a,  Pro-amnion.     a. a,  Araniotic  area,     a.v,  Area  vasculosa.     a. pi,  Area  placentalis.     v.t,  Sinus  terniinalis. — 

(After  van  Beneden  and  Julin.) 

of  the  pharynx,  the  oesophagus,  and  the  stomach.     Underneath  the  posterior 
portion  of  the  fore-gut  there  has  appeared  in  the  mesoderm  a  coelomic  cavity,  p, 


proa 


FIG.  169. — LONGITUDINAL  SECTION  OF  A  YOUNG  CHICK  EMBRYO. 

ff,  Head.  Vd,  Anterior  portion  of  digestive  canal  (Vorderdarm).  mes,  Mesoderm.  fo,  Fovea  cardiaca.  /, 
Pericardial  coelom.  pro.a,  Pro-amnion.  Ach,  Entodermal  cavity,  in  life  bounded  below  by  the  yolk.  Pr.s, 
Primitive  streak. 

which  serves  as  the  connection  across  the  median  line  with  the  amnio-cardiac 
vesicles  just  described  in  surface  views.     We  can,   therefore,   distinguish  in 


E MBR  YO    WITH  SE  VEN  SE  GMENTS. 


299 


the  fore-gut  the  anterior  portion  from  the  posterior  portion  which  overlies  the 
ccelom.  This  coelom  is  the  anlage  of  the  pericardial  cavity.  The  anterior 
division  of  the  fore-gut  forms  the  pharynx  proper.  It  ends  blindly  in  front. 
The  opening  of  the  fore-gut  into  the  general  entodermic  cavity,  Ach,  is  termed 
the  fovea  cardiaca,  fo.  At  the  posterior  end  of  the  embryo  we  have  a  thickened 
mass  of  cells  constituting  the  primitive  streak,  Pr.s.  The  line  on  the  under 
side  of  the  figure  represents  the  entoderm,  and  the  space  underneath  it  is  a 
portion  of  the  primitive  entodermic  cavity. 

Study  of  Transverse  Sections. — Section  through  the  Anterior  Part  of  the 
Head  (Fig.  170). — The  head  lies  free  and  is  covered  by  a  well-defined  layer' of 
ectoderm,  EC,  which  on  the  dorsal  side  is  continuous  with  the  walls  of  the 


TLCh 


md. 


EC 


FIG.  170. — CHICK  EMBRYO  WITH  SEVEN  SEGMENTS.    TRANSVERSE  SECTION  OF  THE  HEAD. 
tick,  Notochord.     Gl,  Ganglionic  or  neural  crest,     md,  Wall  of  medullary  tube,     mes,  Mesoderm.     Ph.,  Pharynx. 
EC,  Ectoderm.     En,  Entoderm.     Am.  V,  Amnio-cardiac  vesicle. 


medullary  canal,  md,  which  is  here  quite  wide,  corresponding  to  the  level  of 
the  future  hind-brain.  Where  the  outer  ectoderm  joins  the  wall  of  the  medul- 
lary canal  there  is  an  accumulation  of  cells,  Gl,  readily  distinguishable  from 
the  adjacent  ectoderm  and  the  medullary  wall  proper.  These  cells  constitute 
the  so-called  neural  crest,  and  represent  the  beginning  of  the  formation  of  the 
true  ganglionic  cells.  On  the  ventral  side  we  have  the  very  widely  distended 
pharynx,  Ph,  which  has  the  characteristic  crescent  shape  in  transverse  sec- 
tions. It  is  lined  by  entoderm  which  forms  a  distinct  continuous  layer,  is  very 
thin  on  the  dorsal  side  and  thicker  on  the  ventral  sideband  on  the  dorsal  side 
is  bent  upward  so  as  to  form  a  median  longitudinal  groove.  The  base  of  this 
groove  touches  the  notochord,  nch,  which  lies  closely  packed  in  the  median 


300 


STUDY  OF  YOUNG  CHICK  EMBRYOS. 


line  between  the  floor  of  the  medullary  canal  and  the  roof  of  the  pharynx. 
The  notochord  presents  a  circular  outline,  and  its  large  size,  as  compared 
with  that  of  the  mammal,  should  be  noted,  for  it  is  characteristic  of  the  Saurop- 
sida.  The  space  between  the  pharynx  and  the  ectoderm  is  partially  filled  by 
loosely  scattered  mesodermic'  cells,-  mes.  •  Underneath  the  head  of  the  embryo 
we  have  two  layers  of  cells  which  belong  respectively  to  the  ectoderm,  EC, 
and  entoderm,  En,  and  together  constitute  the  so-called  .pro-amnion.  On 
either  side,  at  a  point  corresponding  roughly  to  the  lateral  boundary  of  the 
head,  the  two  layers  of  the  pro-amnion  separate  widely  from  one  another  and 
a  third  layer  of  cells  appears  between  them,  which  belongs  to  the  meso.derm 
and  surrounds  the  large  coelomic  cavity  which  is  characteristic  of  the  amnio- 


FIG.  171. — CHICKEN  EMBRYO  WITH  SEVEN  SEGMENTS.    TRANSVERSE  SECTION  THROUGH  THE  ANLAGE  OF 

.    .  THE  HEART. 

Md,  Medullary  groove.     £c,  Ectoderm,     mes,  Mesenchyma.     Am.ves,  Amnio-cardiac  vesicle,      f/i,  Pharynx. 
msf/i,  Mesothelium.     Endo,  Cells  to  form  the  anlage  of  the  endothelial  heart. 

cardiac  vesicles,  Am.V.   'The  letters  in  the   figure   are   placed  in  the  coelom 
of  the  right-hand  vesicle. 

Section  through  the  Posterior  Division  of  the  Fore-gut  (Fig.  171). — In  the  em- 
bryo, from  which  this  section  was  taken  the  medullary  groove,  Md,  was  not  yet 
closed  at  this  point.  At  the  upper  edge  of  the  groove  traces  of  the  differentiation 
of  the  neural  crest,  so  clearly  shown  in  the  previous  figure,  can  be  distinguished, . 
and  the  ectoderm  of  the  groove  at  its  edge  passes  over  into  the  general  ectoderm, 
EC,  covering  the  head  of  the  embryo.  The  fore-gut,  Ph,  the  notochord,  and 
the  mesoderm,  mes,  are  all  very  much  the  same  as  in  the  previous  section. 
On  the  under  side  of  the  embryo  the  structure  is  quite  different.  The  amnio- 
cardiac  vesicles.  Am. yes,  come  very  close  together  in  the  median  line.  They 


EMBRYO   WITH  SEVEN  SEGMENTS. 


301 


are  lined  by  a  distinct  layer  of  mesothelium,  msth,  which,  for  the  most  part, 
is  very  thin,  but  underneath  the  wide  pharynx  the  mesothelium  has  acquired 
considerable  thickness  and  is  somewhat  irregular,  as  shown  in  the  figure.  Be- 
tween the  mesothelium  and  the  floor  of  the  fore-gut.  Ph.  there  lie  a  few  cells, 
Endo,  which  are  the  precursors  of  the  endothelium  of  the  heart;  the  thickened 
mesothelium  forms  the  muscular  wall  of  the  heart.  In  the  figure  there  is  a 
median  partition  of  mesodermic  cells  by  which  the  two  amnio-cardiac  vesicles 
are  separated  from  one  another.  During  this  stage,  or  very  soon  after,  this 
partition  breaks  down  and  disappears,  and  thereafter  the  two  vesicles  com- 
municate freely  across  the  median  line,  and  the  pericardial  chamber  is  said 


FIG.   172. — CHICKEN   EMBRYO,  TRANSVERSE  SECTION  ACROSS  THE  ANLAGE  OF  THE   HEART   IN  A  STAGE 

SLIGHTLY  MORE  ADVANCED  THAN  FlG.   1 71. 

Aid,  Wall  of  medullary  tube,     nch,  Notochord.     msth,  Mesothelium.     Ph,  Pharynx,     pro. am.,  Tip  of  pro- 
amnion.     En.ht,  Endothelial  heart,     m.ht,  Muscular  heart. 

to  be  formed,  though  it  is  not  yet  delimited  from  the  general  coelom  of  the 
vesicles. 

The  further  development  of  the  heart  may  be  understood  by  the  examina- 
tion of  a  somewhat  older  stage  (Fig.  172).  As  shown  in  the  illustration,  the 
mesothelium  has  become  very  protuberant,  m.  hi,  in  the  median  line  under- 
neath the  fore-gut,  Ph.  On  either  side  it  rapidly  thins  out,  msth.  In  the 
protuberant  fold  we  can  recognize  the  future  muscular  heart,  as  it  is  sometimes 
called.  The  few  cells  (Fig.  171,  Endo}  above  described  have  increased  consider- 
ably in  number  and  have  joined  themselves  together  in  such  a  manner  as  to 


302 


STUDY  OF  YOUNG  CHICK  EMBRYOS. 


Seg 


En 


FIG.  173. — CHICKEN  EMBRYO  WITH  SEVEN  SEGMENTS. 

Three  transverse  sections  across  the  caudal  end  of  the  medullary  groove:  A,  Section  through  one  of  the  seg- 
ments ;  B,  Section  posterior  to  the  segments ;  C,  Section  just  in  front  of  the  primitive  streak.  Md.gr, 
Medullary  groove,  nch,  Notochord.  EC,  Ectoderm,  tries,  Mesoderm.  En,  Entoderm.  X  23°  diams. 


EMBRYO   WITH  SEVEN  SEGMENTS. 


303 


indicate  clearly  the  formation  of  the  endothelial  heart  (Fig.  158,  Endo).  At 
first  the  cells 'are  irregularly  disposed  and  have  several  irregular  cavities  between 
them  which  soon  fuse  so  as  to  form  two  main  cavities  running  longitudinally.  As 
the  two  cavities  enlarge  they  meet  in  the  median  line  and  remain  separated  at 
first  by  a  wall  of  two  layers  of  endothelium.  This  wall  soon  breaks  through,  and 
there  results  a  single  median  tube  of  endothelium  which  presently  appears  to  be 
connected  with  the  mesothelium,  m.  ht,  by  long  cell  processes  across  the  wide, 
intervening  space.  The  heart  is  now  a  double  tube  connected  by  the  meso- 
thelium with  the  tissues  above.  Still  later  we  shall  find  the  endothelial  heart 
enlarged  and  the  muscular  heart  to  have  grown  across  on  the  dorsal  side  so  as  to 
form  a  closed  tube  which  separates  finally  on  the  dorsal  side  from  the  mesothe- 
lium with  which  it  is  originally  connected,  and  after  this  separation  we  have  a 
double  free  heart  tube  underlying  the  fore-gut. 


FIG.    174. — CHICKEN    EMBRYO  WITH    SEVEN    SEGMENTS.     TRANSVERSE    SECTION   ACROSS  THE   PRIMITIVE 

GROOVE. 

EC,  Ectoderm,     mes,  Mesoderm.     Ent,  Entoderm.     f>'-g,  Primitive  groove.     The  large  black  dots  represent 

yolk-grains.      X  23°  diams. 

Sections  through  the  Medullary  Groove. — Figure  1 73  represents  three  sections 
at  different  levels.  In  the  first,  A,  the  groove  is  quite  deep  and  the  young 
primitive  segment  is  shown.  At  the  edge  of  the  groove  its  thick  walls  pass  over 
continuously,  but  quite  abruptly,  into  the  general  ectoderm,  EC,  covering  the 
embryo.  Close  under  the  median  line  of  the  medullary  groove  appears  an  oval 
section  of  the  notochord,  nch.  The  entoderm,  En,  is  quite  thin  and  somewhat 
irregular,  as  is  shown  in  all  of  the  sections.  In  B  the  medullary  groove  is  wide 
open  and  quite  shallow,  the  notochord  is  much  larger  and  extends  from  the  floor 
of  the  medullary  groove  to  the  entoderm  and  occupies  in  part  a  deep  notch  in  the 
medullary  wall.  The  notochord  prevents  the  extension  of  the  mesoderm  across 
the  median  line.  In  C  the  medullary  groove  is  fading  out  and  merging  into  the 
beginning  of  the  primitive  streak,  which  forms  a  large  mass  of  cells  in  the  median 
line  in  which  the  boundaries  between  the  gerni-layers  cannot  be  determined. 
Laterally  this  mass  of  tissue  passes  over  into  perfectly  distinct  germ-layers,  of 


304  STUDY  OF  YOUNG   CHICK  EMBRYOS. 

which  the  middle  or  mesoderm,  mes,  is  by  far  the  most  voluminous.  The  walls 
of  the  medullary  groove  are  crowded  with  nuclei  which  lie  at  every  possible  level, 
some  close  to  the  inner,  others  close  to  the  outer  surface,  and  also  in  every  possi- 
ble intervening  position.  The  nuclei  are  much  crowded,  there  being  but  little 
protoplasm.  'No  distinct  cell  boundaries  can  be  made. out.  The  nuclei  are 
further  remarkable  on  account  of  their  very  conspicuous  nucleoli. 

Section  through  the  Primitive  Groove  (Fig.  174). — The  section  passes  through 
the  anterior  region  of  the  groove.  Underneath  the  groove,  which  appears  in 
the  section  as  a  shallow  notch,  Pr.g,  the.  germ-layers  are  fused  with  one  another 
and  show  no  recognizable  boundary;  laterally,  however,  they  become  entirely 
distinct.  The  ectoderm,  EC,  is  quite  thick,  representing,  presumably,  the  stage 
of  the  embryonic  shield  (compare  page  62).  The  mesoderm,  mes,  is  of  about 
the  same  thickness  as  the  ectoderm,  but  its  cells  are  far  less  compactly  crowded. 
In  the  median  line  it  also  merges  without  boundary  into  the  tissue  of  the  primi- 
tive streak.  The  entoderm,  Ent,  is  very  thin  indeed  and  makes  a  striking  con- 
trast with  the  appearance  in  the  same  layer  in  the  region  "of  the  area  opaca 
(compare  Fig.  35). 


CHAPTER  VI. 

STUDY   OF   THE    BLASTODERMIC  VESICLE   AND   THE 
SEGMENTATION  OF  THE  OVUM. 

Method  of  Obtaining  Blastodermic  Vesicles  from  the  Rabbit. 

The  does  should  be  allowed  to  become  pregnant  and  be  isolated  until 
they  have  littered;  the  date  of  littering  should  be  noted,  and  thirty  days  there- 
after the  buck  be  admitted  and  the  exact  time  of  the  covering  recorded.  At  the 
proper  number  of  days  thereafter  the  animal  should  be  killed  and  the  uterus 
removed  at  once.  It  may  be  opened  with  two  pairs  of  forceps  used  to  split  the 
outer* muscular  walls  of  the  organ,  beginning  the  operation  at  the  lower  end  of 
the  uterus:  With  a  little  care  this  can  be  done  without  rupturing  the  mucous 
membrane,  which  is  to  be  afterward  also  opened  in  a  similar  manner  with  the 
forceps  and  the  blastodermic  vesicles  exposed.  They  are  small  bodies  of  rounded 
form  and  with  a  brilliant  pearly  luster,  and  are  easily  observed.  During  the 
earlier  stages,  which  occur  in  the  Fallopian  tubes,  the  ova  are  very  small  and 
difficult  to  find,  but  by  the  time  the  ovum  has  reached  the  uterus  it  has  become 
a  blastodermic  vesicle  measuring  about  0.6  mm.  in  diameter,  and,  therefore, 
easily  seen  by  the  naked  eye.  From  the  fourth  day  after  coitus  until  the  begin- 
ning of  the  seventh  day  the  vesicles  lie  free  in  the  uterus.  Usually  early  in  the 
seventh  day  the  vesicles,  which  then  measure  about  4.5  by  3.5  mm.,  begin  to 
attach  themselves  to* the  wall  of  the  uterus,  and  thereafter  are  much  more  diffi- 
cult to  remove.  At  the  beginning  of  the  fifth  day  the  ova  measure  about  0.6  to 
0.9  mm.  in  diameter,  but  vary  greatly  in  size,  and  are  found  more  or  less  near 
together  in  the  upper  portion  of  the  oviduct.  By  the  end  of  the  sixth  day  they 
measure  about  4.0  mm.  and  are  distributed  throughout  the  entire  length  of  the 
uterus. 

The  most  useful  stages  are  the  vesicles  from  the  beginning  of  the  sixth  and 
seventh  days.  To  preserve  the  vesicles  they  must  be  gently  removed  from  the 
uterus,  great  care  being  necessary  not  to  injure  them,  and  dropped  into  Zenker's 
or  Hermann's  fluid.  In  either  of  these  they  may  be  left  for  about  an  hour  and 
then  washed  and  preserved  in  the  usual  manner.  Specimens  should  be  examined 
in  the  fresh  state,  just  after  they  have  been  preserved,  and  after  they  have  been 

20  305 


306  STUDY  OF  THE  BLASTODERMIC  VESICLE. 

\ 

stained  before  they  are  imbedded.  For  staining,  alum  cochineal  or  borax  car- 
mine is  recommended.  Finally,  the  specimens  are  to  be  imbedded  in  paraffin 
and  cut  in  series  in  the  usual  manner ;  sections  of  from  6  to  8  n  are  desirable.  Un- 
fortunately no  method  has  yet  been  devised  by  which  these  delicate  vesicles  may 
be  imbedded  without  distortion  of  their  form,  so  that,  when  the  sections  are 
finally  obtained,  the  blastodermic  walls  are  wrinkled  and  more  or  less  out  of 
shape.  But  fortunately,  owing  apparently  to  its  greater  thickness,  the  em- 
bryonic area  usually  escapes  distortion  and  appears  in  the  sections  of  normal 
form,  or  nearly  so. 

Study  of  Rabbit  Blastodermic  Vesicles  in  Alcohol. 

All  of  the  most  important  points  in  the  structure  of  the  blastodermic  vesi- 
cles of  the  rabbit  from  the  fourth  to  the  seventh  day  may  be  fairly  well  observed 
by  examining  the  hardened  vesicles  in  alcohol  under  the  microscope.  For  such 
examinations  the  so-called  live-box,  such  as  was  formerly  much  used  by  micro- 
scopists  for  the  study  of  living  creatures,  will  be  found  very  convenient.  Care 
must  be  taken  to  have  plenty  of  alcohol  around  the  specimen  and  not  to  lower 
the  cover  so  much  as  to  exert  any  pressure  upon  the  vesicle.  It  is  not  difficult  to 
place  the  vesicles  so  that  any  part  of  their  surface  may  be  examined  with  a  No.  7 
objective.  In  the  uncolored  specimen  the  nuclei,  and  even  many  of  the  bounda- 
ries of  the  cells,  can  be  clearly  made  out. 

In  the  following  descriptions  ages  have  been  chosen  at  which  the  important 
characteristics  can  usually  be  observed.  The  variation  is  so  great  in  range  dur- 
ing early  stages  that  the  development  described  below  for  a  given  age  is  often 
found  in  older  or  younger  specimens,  and  specimens  of  a  given  age  may  exhibit  a 
less  or  a  more  advanced  stage  of  the  embryonic  formation  than  is  here  put  down 
for  that  age.  In  general  the  correspondence  of  the  stage  of  development  to  the 
size  of  the  vesicle  is  more  exact  than  to  its  age. 

Vesicles  at  Five  Days  (5  X  24  hours). — At  this  age  the  vesicles  are  always 
found  in  the  upper  portion  of  the  uterus.  Sometimes  all  of  those  in  one  uterus 
are  quite  close  together,  at  other  times  somewhat  scattered  and  lying  singly. 
The  vesicles  are  extremely  variable  in  size,  for  they  measure  from  0.6  to  0.9  mm. 
They  are  spherical  or  nearly  so,  and  are  surrounded  by  a  thin  membrane,  which 
in  reality  corresponds  to  both  the  zona  pellucida  and  the  outer  albuminous  en- 
velope, which  in  the  rabbit  ovum  during  segmentation  is  very  thick  and  con- 
spicuous, but  which  is  always  extremely  thin  when  the  stage  of  the  blastodermic 
vesicle  is  reached.  Upon  the  outside  of  this  really  double  membrane  appear  a 
certain  number  of  small  villus-like  projections,  which  are  highly  refringent. 
They  are  probably  identical  in  character  with  the  villi  which  have  been  observed 


RABBIT  VESICLES  IN  ALCOHOL.  307 

upon  the  ovum  of  the  dog  (pages  59,  and  60),  but  are  smaller  in  all  of  their  dimen- 
sions. Immediately  underneath  the  external  membrane  there  is  a  continuous 
layer  of  cells  belonging  to  the  ectoderm  and  extending  completely  around  the 
ovum.  The  layer  is  sometimes  designated  specifically  as  the  "  outer  layer"  or  as 
the  ' '  subzonal  layer. ' '  It  also  extends  over  the  embryonic  shield ;  the  portion  upon 
the  shield  is  often  termed  Rauber's  layer,  it  having  been  first  observed  by  that 
investigator.  The  cells  of  the  outer  layer  are  quite  large  and  their  boundaries 
are  easily  recognizable  in  surface  views.  Their  sides  may  number  four,  five,  or 
six,  six  being  perhaps  the  more  usual  number,  and  are  variously  disposed,  so  that 
the  cells  differ  in  shape  and  size.  During  the  next  two  days  of  development  the 
cells  become,  if  anything,  more  irregular  in  outline  and  somewhat  smaller.  The 
boundaries  between  the  cells  are  very  fine  lines ;  the  nuclei  are  rather  large  and 
oval  in  form,  and  contain  from  three  to  four  or  five  highly  refringent  granules. 
Each  nucleus  is  surrounded  by  a  denser  court  of  protoplasm  in  which  there  are 
many  granules,  some  of  which  are  highly  refringent.  The  peripheral  portion  of 
the  cell  is  of  a  loosely  reticulate  structure  with  comparatively  wide  meshes  be- 
tween the  threads  of  the  protoplasm.  Occasionally  there  appear  in  the  proto- 
plasm of  these  cells  narrow,  elongated,  highly  refringent  bodies  somewhat  re- 
sembling bacilli  in  appearance,  and  therefore  they  are  termed  the  bacillijorm 
bodies.  Their  nature  is  unknown ;  they  are  more  apt  to  be  found  in  older  vesi- 
cles. The  outer  or  subzonal  layer  can  be  made  out  over  the  embryonic  shield 
only  by  very  careful  observation.  In  the  shield  the  cells  are  several  layers 
thick.  The  inner  cells  are  very  much  smaller  in  size  than  the  cells  of  the  outer 
layer,  are  more  granular,  and  contain  smaller  nuclei  which  take  up  a  relatively 
large  place  in  the  cell  in  proportion  to  its  apparent  area.  Closer  observation, 
utilizing  the  fine  adjustment  of  the  microscope,  will  show  that  there  are  two 
kinds  of  cells  in  the  inner  part:  first,  those  which,  like  the  cells  of  the  subzonal 
layer,  belong  to  the  ectoderm;  and,  second,  an  inner  layer  of  cells,  which  appar- 
ently belongs  entirely  to  the  entoderm.  In  the  region  of  the  embryonic  shield 
the  ectoderm  is,  therefore,  made  up  of  two  distinct  layers  of  cells.  The  outer  or 
subzonal  (Rauber's  layer)  disappears  during  the  sixth  day  of  development  as  a 
distinct  layer.  The  cells  of  the  entoderm  form  a  very  thin  continuous  layer  on 
the  under  side  of  the  embryonic  shield.  They  may  be  recognized  by  the  very 
granular,  and  therefore  dark,*  appearance  of  their  protoplasm,  and  by  the 
rounded  form  and  small  size  of  their  nuclei.  Similar  cells  may  be  observed  also 
extending  beyond  the  limits  of  the  embryonic  shield,  though  not  there  forming  a 
continuous  layer,  except  perhaps  for  a  very  short  distance,  but  rather  lying 

*  As  seen  by  transmitted  light. 


308  STUDY  OF  THE  BLASTODERMIC  VESICLE. 

scattered  about  in  patches  or  isolated.  As  the  cuboidal  cells  of  the  ectoderm 
are  confined  to  the  region  of  the  embryonic  shield,  the  cells  of  the  entoderm 
outside  of  the  shield  lie  close  against  the  subzonal  layer.  Here  they  may  be 
more  easily  studied  than  in  the  shield  itself.  They  are  very  much  smaller  than 
the  cells  of  the  outer  layer  and  contain  each  a  nucleus  with  highly  refringent 
granules,  which  are  now  numerous  and  smaller  than  the  somewhat  similar 
granules  in  the  overlying  nuclei  of  the  ectoderm.  The  further  away  we  proceed 
from  the  edge  of  the  embryonic  shield,  the  fewer  we  find  the  entodermal  cells. 
The  extent  of  their  distribution  varies  greatly,  and  apparently  more  or  less  in 
relation  to  the  size  of  the  blastodermic  vesicle,  since  in  the  smallest  vesicles  of 
this  age  we  find  the  cells  only  a  short  distance  beyond  the  edge  of  the  shield,  yet 
in  the  largest  vesicles  they  have  expanded  even  past  the  equator. 

Vesicles  at  Six  Days. — At  this  age  the  vesicles  are  found  more  or  less  scat- 
tered and  isolated  in  position  from  one  another  through  the  upper  half  of  the 
uterus.  They  measure  from  i  .o  to  1.6  mm. ,  their  walls  are  very  transparent,  and 
the  somewhat  more  opaque,  round  or  oval  embryonic  shield  can  be  readily  distin 
guished  with  a  hand  lens.  Its  size  varies  with  the  diameter  of  the  vesicle,  being 
larger  in  the  larger  vesicles;  but  the  proportions  are  not  exact,  for  a  vesicle  of 
given  diameter  may  have  an  embryonic  shield  of  either  larger  or  smaller  dimen- 
sions than  other  vesicles  of  the  same  size.  Hence,  vesicles  of  different  sizes  may 
have  embryonic  shields  of  similar  dimensions.  The  actual  diameter  of  the  shield 
is  between  0.2  and  0.35  mm.  The  general  structure  of  the  vesicles  is  the  same 
as  at  five  days,  but  certain  differences  may  be  noted.  In  preserved  specimens 
the  external  membrane  is  very  apt  to  be  wrinkled.  The  subzonal  layer  has  very 
much  the  same  appearance  as  before,  though  the  cells  are  somewhat  smaller  and 
it  has  almost  disappeared  over  the  region  of  the  embryonic  shield.  The  manner 
of  its  disappearance  has  not  been  definitely  settled.  There  is  no  evidence  that 
the  cells  degenerate  or  are  cast  off,  hence  one  inclines  to  the  hypothesis  that  the 
cells  of  the  subzonal  layer  become  incorporated  in  the  inner  layer  of  the  cuboidal 
ectodermal  cells,  for  in  sections  shown  at  this  stage  the  ectoderm  is  one-layered 
in  the  region  of  the  shield.  Entodermal  cells  also  have  essentially  the  same 
appearance  as  at  five  days,  but  they  extend  considerably  further  around  the 
vesicle,  are  more  numerous,  and  form  a  more  continuous  layer.  Sections  show 
that  the  subzonal  layer  outside  of  the  shield  is  very  thin,  but  its  outer  surface  is 
fitted  to  the  inner  surface  of  the  zona  pellucida.  The  center  of  each  cell  is  some- 
what thicker,  projecting  toward  the  interior  of  the  vesicle.  It  is  in  this  thicker 
projecting  portion  that  the  nucleus  is  situated.  Along  the  borders  of  the  cells  the 
layer  is  of  course  thinner,  and  it  is  under  these  thinner  parts  that  the  thicker 
nucleated  portions  of  the  entodermal  cells  are  lodged.  Hence,  in  surface  views, 


RABBIT  VESICLES  IN  ALCOHOL.  309 

the  nuclei  of  the  two  layers  are  seen  to  alternate  more  or  less  with  one  another. 
This  characteristic  disposition  is  not  kept  everywhere,  but  is  subject  to  consider- 
able variations.  In  the  very  most  advanced  ova  of  six  days  a  small  spot  some- 
times can  be  observed  in  the  embryonic  shield  which  is  noticeable  from  its 
greater  opacity.  This  spot  corresponds  to  Hensen's  knot,  but  it  does  not  usually 
show  itself  distinctly  until  considerably  later. 

Vesicles  at  Seven  Days. — Vesicles  at  this  age  vary  greatly  in  size,  and  the 
stage  of  development  varies  with  the  size — how  exactly,  we  do  not  yet  know. 
Preliminarily  we  may  fix  on  the  normal  size  as  being  that  of  vesicles,  the  greatest 
diameter  of  which  is  about  4  mm.  Such  vesicles  are  somewhat  oval  in  shape  and 
slightly  flattened  on  the  side  bearing  the  embryonic  shield.  The  membrane 
enclosing  them  is  very  thin ;  the  albuminoid  layer  can  scarcely  be  distinguished, 
but  the  zona  pellucida  is  very  distinct.  The  shield  (Fig. 
175,  Sh)  is  somewhat  elongated  and  distinctly  pear- 
shaped.  Its  long  axis  is  parallel  with  that  of  the  vesicle. 
It  varies  greatly  in  its  dimensions.  Shields  i  mm.  wide, 
and  from  1.3  to  1.4  mm.  long,  are  not  uncommon.  The 
student  will  be  likely  to  encounter  other  dimensions.  The 
most  striking  addition  is  the  appearance  of  a  darker  area, 
mes,  at  the  posterior  or  pointed  end  of  the  shield.  This 
darker  area  is  also  somewhat  pear-shaped,  but  its  pointed  FlG  -  _BLASTODERMIC 
end  is  near  the  center  of  the  shield,  its  rounded  end  a  little  VESICLE  OF  A  RABBIT 

distance  behind  the  point  of  the  shield.     The  darker  area  AT  SEVEN  DAYS- 

owes  its  formation  to  the  appearance  of  a  new  layer  of  SA>  Embry°nic  shield,  mes, 
cells  between  the  ectoderm  and  entoderm.  This  layer  grammatic  ) 

consists  of  loosely  connected  cells  with  rounded  nuclei 

easily  distinguishable  in  surface  views  from  those  of  the  subzonal  layer.  The 
greater  part  of  these  cells  are  certainly  mesodermic,  but  a  portion  of  them  share 
in  the  formation  of  the  primitive  streak  and  notochordal  canal,  and  perhaps  do 
not  belong  to  the  mesoderm.  In  the  region  outside  the  embryonic  shield  the  outer 
layer  is  easily  distinguished ;  its  cells  have  marked  outlines,  but  are  of  smaller 
dimensions  than  in  earlier  stages,  their  nuclei  are  large,  for  the  most  part  oval, 
and  contain  several  highly  refringent  and  conspicuous  granules.  The  number 
of  granules  varies ;  when  there  are  only  two  or  three,  they  are  apt  to  be  elongated 
as  if  several  small  granules  had  united.  The  entodermal  cells  have  spread  well 
past  the  equator  of  the  vesicle  and  present,  for  the  most  part,  a  distinctly  epi- 
thelial arrangement,  although  at  the  edge  of  the  expanding  layer  the  cells  are  still 
more  or  less  scattered.  The  entodermal  cells  are  easily  distinguished  by  chang- 
ing the  focus  of  the  microscope,  when  their  darker  protoplasm  and  smaller  size, 


310  STUDY  OF  THE  BLASTODERMIC  VESICLE. 

together  with  their  smaller  darker  nuclei,  make  them  readily  recognizable.  The 
granules  in  the  entodermic  nuclei  are  smaller  and  more  numerous  than  in  the 
overlying  ectodermal  nuclei. 

During  the  next  few  hours  further  changes  ensue.  At  the  apex  of  the  pear- 
shaped  mesodermic  area  there  appears  a  small  spot,  which  is  known  as  Hensen's 
knot.  At  first  Hensen's  knot  consists  of  a  little  thickening  accompanied  by  a 
union  of  the  cells  of  the  middle  layer  with  those  of  the  overlying  ectoderm.  Next 
occurs  the  development  of  the  primitive  streak,  which  runs  from  Hensen's  knot 
backward  toward  the  apex  and  embryonic  shield,  and  very  soon  thereafter  along 
the  line  of  the  primitive  streak  there  develops  the  external  and  shallow  primitive 
groove.  At  Hensen's  knot  the  three  layers  now  are  found  to  be  intimately 
united,  so  that,  though  they  may  everywhere  else,  when  fresh,  be  separated  from 
one  another,  the  germ-layers  at  this  point  cannot  be  separated,  except  by  tearing. 
Finally,  in  the  next  stage  there  grows  out  in  front  of  Hensen's  knot  the  so-called 
head  process,  an  axial  band  of  cells  in  which  the  notochordal  canal  develops. 


Ent. 


FIG.    176.  —  RABBIT   EMBRYO  OF  SEVEN   DAYS.     TRANSVERSE  SERIES   12,  SECTION   216,   THROUGH   THE 

ANTERIOR  PORTION  OF  THE  EMBRYONIC  SHIELD. 

EC,  Ectoderm.     Ent,  Entoderm.     X  3°°  diams. 

The  structure  of  the  embryonic  shield  at  seven  days  is  further  illustrated  by 
the  two  sections  represented  in  figures  176  and  177,  the  former  passing  across 
the  anterior  portion  of  the  shield,  where  it  is  two-layered  ;  and  the  latter  across 
the  posterior  portion,  in  which  the  middle  layer  has  appeared.  Figure  1  76  shows 
the  middle  portion  of  a  section.  It  consists  merely  of  the  outer,  thicker,  ecto- 
dermar  layer,  EC,  and  the  very  thin  entodermal  layer,  Ent.  Both  surfaces  of 
the  ectoderm  are  quite  sharply  defined.  The  nuclei  are  rather  large  and  show 
several  large,  deeply  stained  nucleoli  in  each.  The  outline  of  the  nucleus  is 
sharp,  and,  in  addition  to  the  larger  granules,  there  are  many  smaller  ones  less 
deeply  stained  scattered  through  the  nucleus.  The  nuclei  vary  considerably 
in  size,  shape,  and  position.  The  protoplasm  of  the  ectodermal  cells  is  lightly 
stained,  and  granular  in  appearance.  The  boundaries  between  adjacent  cells  are 
indicated  by  delicate  lines,  which  extend  through  the  entire  thickness  of  the 
ectoderm,  which  is  now  but  a  single  layer  of  cells.  The  original  outer  (Rauber's) 
layer  has  disappeared.  The  entoderm  is  very  thin,  but  is  thickened  a  little 


RABBIT  VESICLES  OF  SEVEN  DAYS. 


311 


where  each  nucleus  is  lodged.  The 
nuclei  are  smaller  than  those  of  the 
ectoderm,  more  darkly  stained,  and 
the  granules  in  them  less  coarse  than 
those  in  the  nuclei  of  the  ectoderm. 
Between  the  two  layers  is  a  narrow 
space;  whether  an  artefact  or  not  is 
difficult  to  say.  Figure  177  represents 
a  transverse  section  through  the  poste- 
rior part  of  the  embryonic  shield;  the 
position  of  the  median  plane  is  ap- 
proximately indicated  by  the  vertical 
line  M.  About  this  plane  there  is  a 
considerable  accumulation  of  cells  which 
merges  without  boundary  into  the  sup- 
erficial cells  of  the  shield.  A  short  dis- 
tance from  the  median  line  the  outer 
layer  of  the  shield  becomes  a  distinct 
epithelium,  EC,  consisting  of  a  single 
layer  of  cells.  The  edge  of  the  shield 
is  marked  by  a  rather  abrupt  transition 
to  the  thin  outer  layer  of  the  extra- 
embryonic  region.  On  the  under  side 
of  the  section  extends  the  thin  ento- 
derm  as  a  continuous  layer,  which  is 
only  loosely  connected  with  the  central 
mass  of  cells  overlying  it  near  the  me- 
dian plane.  Finally,  from  the  median 
mass  of  cells  extends  laterally  the  sheet 
of  mesoderm,  Mes,  between  the  outer 
and  inner  germ-layers.  The  mesoder- 
mic  cells  are  somewhat  loosely  dis- 
tributed, and  have  round  nuclei  with 
distinct  chromatin  granules  and  well- 
marked  protoplasmic  bodies,  which 
give  off  strands  by  which  the  cells 
are  united  to  one  another.  The  middle 
germ-layer  is  the  least  compact  of  the 
three. 


X 


d 
-  .j    <u 

ITS  -g 


312  THE  SEGMENTATION  OF  THE  OVUM. 

The  Maturation,  Fertilization,  and  Segmentation  of  the  Ovum  in  White  Mice. 

These  animals  are  selected  for  the  practical  study  of  the  earliest  stages  of 
development  for  two  reasons:  first,  because  the  processes  have  been  more  thor- 
oughly studied  in  them  than  in  any  other  mammals;  and,  second,  because  they 
are  easily  kept  and  breed  freely,  so  that  abundant  material  may  be  secured  with 
comparatively  little  trouble.  Those  desiring  further  information  are  referred 
to  Sobotta's  original  memoir.* 

Heat  occurs  twenty-one  days  after  littering,  a  fact  which  may  be  taken 
advantage  of  to  secure  ova  of  the  desired  age.  Coitus  can  take  place  only  during 
heat,  for  it  is  then  only  that  the  vagina  is  found  open;  at  other  times  its  epithe- 
lium concresces  to  a  solid  mass.  The  spermatozoa  do  not  penetrate  into  the 
tube  until  some  time  after  the  coitus.  After  the  discharge  of  the  semen,  the 
contents  of  the  large  seminal  vesicle  are  ejaculated  into  the  vagina,  completely 
filling  it  and  hardening  into  a  white  plug  (bouchon  vaginal),  as  in  guinea-pigs. 
From  twenty  to  thirty  hours  later  the  plug  softens  and  falls  out. 

The  Fallopian  tubes  are  narrow,  much  contorted  canals.  The  fimbriate 
opening  of  the  tube  penetrates  the  connective  tissue  about  the  ovum  so  that  the 
fimbriae  lie  in  the  periovarial  space.  There  is  ciliated  epithelium  in  the  proximal 
region  of  the  tube  only,  none  in  the  distal  parts  or  in  the  uterus  itself.  During 
heat  the  periovarial  space  is  filled  with  an  abundant  clear  fluid.  This  also  dis- 
tends the  proximal  part  of  the  tube,  forming,  as  it  were,  a  special  sac,  with  a  dis- 
tended epithelial  lining.  At  the  time  of  coitus  ovulation  has  generally  taken 
place;  the  ovum,  still  surrounded  by  the  cells  of  the  corona  radiata,  is  found  in 
the  fluid  of  the  distended  proximal  section  of  the  tube.  It  is  probable  that  the 
ova  are  carried  from  the  periovarial  space  not  only  by  the  currents  created  by  the 
cilia  of  the  fimbriate  opening,  but  also  by  a  sort  of  pumping  action  of  the  tube 
itself.  For  at  the  beginning  of  the  period  of  heat  we  find  that  the  periovarial 
space  contains  much  fluid,  but  later,  when  the  ova  are  in  the  tube,  this  space  is 
empty  and  the  tube  contains  fluid.  The  ovum  of  the  mouse  measures  only  59  // 
in  diameter,  and  is,  therefore,  the  smallest  known  mammalian  ovum.  It  is 
surrounded  by  a  very  thin  zona  pellucida  measuring  about  1.2  /*,  and  contains 
only  a  few  yolk-grains,  a  portion  of  which  may  be  blackened  by  osmic  acid. 
These  ova  offer  the  further  special  peculiarity  that  they  sometimes  form  two 
and  sometimes  (from  80  to  90  per  cent,  of  the  cases)  only  a  single  polar  globule. 
When  two  are  formed,  the  first  appears  while  the  cell  is  still  in  the  ovary,  the 
second  after  it  has  been  transferred  to  the  Fallopian  tube;  when  only  one  is 


*"Die  Befruchtung  und  Furchung  des   Eies  der  Maus,"  "  Archiv  f.  mikroskopische  Anatomic,"  vol. 
XLV,  pp.  15-93,  Pis.  II-IV  (1895). 


POLAR  GLOBULES  IN  WHITE  MICE. 


313 


formed,  it  is  developed  in  the  ovum  as  it  lies  in  the  tube.  When  two  globules  are 
formed,  the  nuclear  spindles  of  the  first  and  second  differ  somewhat  in  appear- 
ance; when  only  one  is  formed,  it  resembles  that  of  the  second  globule.  Hence 
we  are  led  to  surmise  that  there  may  have  been  in  these  cases  really  a  first  polar 
globule  formed,  but  that  it  appeared  so  much  earlier  in  the  history  of  the  cell  that 
it  escaped  observation. 

The  First  Polar  Globule. — The  first  polar  globule  is  formed,  as  stated,  while 
the  ovum  is  still  in  the  unruptured  Graafian  follicle  of  the  ovary.  The  nucleus 
moves  toward  one  side  of  the  ovum  and  is  there  transformed  into  a  mitotic 
spindle,  the  axis  of  which  is  more  or  less  nearly  at  right  angles  to  the  radius  of  the 
ovum  (Fig.  178).  The  spindle  itself  is  large,  pointed  at  the  ends,  with  curving 
achromatic  threads.  The  chromosomes,  which  are  probably  twelve  in  number, 
gather  themselves  into  an  equatorial  plate.  They  are  elongated,  pointed  at  the 
ends,  with  irregular  sides,  and  are  very  large.  No  trace 
of  the  centrosome  has  been  observed  at  the  end  of  the 
spindle  and  there  are  no  astral  rays  extending  from  the 
ends  of  the  spindle  into  the  protoplasm.  The  chrom- 
osomes are  somewhat  V-shaped.  They  divide  by  a 
transverse  separation  at  the  apex  of  the  V.  Chrom- 
osome halves  migrate  toward  the  end  of  the  spindle. 
These  stages  occur  probably  about  twenty-four  hours 
before  the  rupture  of  the  follicle.  The  actual  extrusion 
of  the  first  globule  has  not  yet  been  described.  It 
takes  place  before  ovulation,  for  the  first  polar  globule 
is  always  found  while  the  ovum  is  still  in  the  ovary. 
In  the  mouse  it  is  remarkable,  as  is  also  the  second 

polar  globule,  for  its  large  size.  It  is  usually  oval  or  spherical  in  form,  and  may 
measure  say  16  or  17  /JL  in  length  by  9  ;*  in  diameter.  It  has  a  distinct  cell 
membrane,  a  protoplasm  which  resembles  that  of  the  ovum,  and  may  even  con- 
tain granules  of  yolk.  Soon  after  its  separation  from  the  ovum  its  nucleus  be- 
comes well  developed  and  membranate.  Except,  therefore,  that  the  number  of 
chromosomes  which  enter  into  its  formation  is  half  the  normal  number,  we 
might  say  that  it  differs  little  from  an  ordinary  cell. 

The  Second  Polar  Globule. — After  the  formation  of  the  first  polar  globule  we 
do  not  find  the  nucleus  of  the  ovum  entering  into  a  condition  of  repose,  but  it  at 
once  transforms  itself,  as  in  other  animals,  into  the  second  polar  spindle.  The 
form  and  changes  through  which  this  passes  are  similar  to  those  of  the  spindle 
in  the  case  in  which  only  a  single  globule  is  found,  to  the  description  of  which  we 
now  pass. 


FIG.  178. — OVUM  OF  WHITE 
MOUSE,  WITH  THE  FIRST 
POLAR  SPINDLE  IN  TAN- 
GENTIAL POSITION.  X  5°° 
diams. — ( After  J.  Sobotta. ) 


314 


THE  SEGMENTATION  OF  THE  OVUM. 


The  Single  Polar  Globule. — As  stated  above,  when  only  a  single  polar  globule 
is  formed,  it  appears  in  the  ovum  in  the  ampulla  of  the  tube.  The  nucleus  moves 
toward  the  surface,  loses  its  membrane,  and  produces  a  polar  spindle,  which  is 
much  smaller  than  that  previously  described.  It  lies  at  right  angles  to  the  axis 
of  the  ovum  and  quite  close  to  the  surface.  It  contains  twelve  thick,  achro- 
matic fibers,  which  do  not  unite  at  the  poles  with  one  another,  but  end  parallel, 
so  that  the  tip  of  the  spindle  is  blunted.  The  chromosomes,  when  the  membrane 
first  disappears,  lie  irregularly,  but  shortly  after  the  formation  of  the  spindle  they 
collect  together  to  form  an  equatorial  plate,  somewhat  as  in  figure  178. 
They  are  irregular  and  of  uneven  size,  twelve  in  number,  or  possibly  the  number 
may  vary  somewhat.  The  chromosomes  then  divide  transversely,  the  halves 
move  rapidly  toward  the  ends  of  the  spindle,  which  during  this  change  passes  into 
the  radial  position  (Fig.  179).  The  surface  of  the  ovum  or  the  apex  of  the 


FIG.  179.— OVUM  OF  WHITE  MOUSE,  DIVIDING  TO 
PRODUCE  THE  POLAR  GLOBULE.  X  5°° 
diams. — (Affer  J.  Sobotfa.)  The  elongated 
male  pro-nucleus  lies  in  the  inferior  protuber- 
ance of  the  ovum. 


FIG.  180. — OVUM  OF  WHITE  MOUSE,  SHOWING 
THE  METAPHASE  OF  THE  DIVISION  PRODUC- 
.ING  THE  FIRST  POLAR  GLOBULE.  X  I5°° 
diams. —  (After  J.  Sobotta.} 


spindle  forms  a  protuberance,  and  the  spindlemoves  partly  into  this  protuberance. 
Division  of  the  achromatic  fibers  takes  place,  and  there  is  formed  a  well-marked 
cell  plate  (Fig.  180),  and  presently  the  polar  globule  becomes  constricted  off. 
The  cell  plate  appears  with  unusual  distinctness.  It  is  at  about  this  stage  that 
the  spermatozoon  is  found  to  have  entered  the  ovum  (Fig.  179),  and  to  have 
formed  there  the  male  pro-nucleus.  During  all  these  stages  no  centrosome 
appears  at  the  poles  of  the  spindle,  and  no  astral  rays  appear  in  the  protoplasm, 
although  in  many  eggs  these  astral  figures  are  extremely  conspicuous.  The  nu- 
clear elements  in  the  ovum  proper  appear  at  first  as  a  dense  cluster  of  chromatin 
granules.  These  fuse  apparently  into  a  compact  mass,  which  grows  rapidly  in 
size,  presumably  by  the  absorption  of  fluid  from  the  yolk,  and,  as  it  enlarges, 
acquires  a  distinct  membrane,  and  presently  shows  a  network  structure  in  its 


FERTILIZATION  OF  OVUM  IN  WHITE  MICE. 


315 


interior  (Fig.  182),  with  irregular  chromatin  masses.  It  continues  to  grow  more 
and  more,  and  develops  at  the  same  time  a  series  of  nucleoli  more  or  less  uniform 
in  size.  This  stage  may  be  regarded  as  that  of  the  completed  female  pro-nucleus. 


FIG.  181. — OVUM  OF  WHITE  MOUSE,  AFTER  THE 
FORMATION  OF  THE  POLAR  GLOBULE.  BOTH 
PRO-NUCLEI  ARE  PRESENT.  X  5°°  diams. — 
(After  J.  Sobotta.} 


FIG.  182. — OVUM  OF  WHITE  MOUSE,  WITH  Two 
WELL-DEVELOPED  PRO-NUCLEI.  X  5°°  diams. 
— (After  J.  Sobotta.~) 


Fertilization. — As  the  general  account  of  the  fertilization  or  impregnation 
of  the  mammalian  ovum  is  based  on  the  process  in  the  white  mouse,  it  is  neces- 
sary only  to  refer  to  descriptions  and  figures,  pages  49  to  54. 


CHAPTER  VII. 

STUDY   OF   THE    UTERUS   AND   THE    FCETAL   APPEN- 
DAGES   OF   MAN. 

Histology  of  the  Uterus. 

In  most  mammals  the  uterus  is  double.  This  is  the  case  in  the  pig,  the 
rabbit,  and  the  mouse,  the  three  species  which  furnish  material  for  the  practical 
study  as  planned  in  this  book.  In  these  animals  each  uterus  is  a  long,  more  or 
less  cylindrical  tube.  In  primates  the  double  uterus  exists  only  during  very 
early  embryonic  stages,  after  which  the  two  are  found  united  into  a  single  median 
uterus.  The  mammalian  uterus  is  always  lined  by  a  mucous  membrane,  con- 
sisting of  a  superficial  epithelium,  which  forms  glands,  and  of  a  deeper  layer  of 
reticulate  connective  tissue,  in  which  there  are  lymph  spaces,  nerves,  and  a  fairly 
abundant  blood-supply.  The  mucous  membrane  is  subject  to  very  marked 
periodic  changes  in  structure.  It  is  enclosed  by  the  muscular  layers  of  the  organ, 
the  muscle-fibers  being  of  the  smooth  type.  In  animals  with  double  uteri  the 
muscle-fibers  form  two  distinct  layers,  an  inner  circular  and  an  outer  longitudinal 
layer.  In  the  primate  uterus  the  disposition  of  the  fibers  is  far  more  compli- 
cated, and  the  two  distinct  layers  cannot  be  identified.  The  surface  of  the  uterus, 
wherever  it  is  free,  is  covered  by  a  layer  of  peritoneum  which  consists  of  a  layer 
of  flattened  epithelial  cells  and  a  thin  underlying  layer  of  fibrillar  connective 
tissue. 

The  human  uterus  at  birth  has  a  mucosa  which  is  about  0.2  mm.  thick. 
The  mucosa  is  soft,  pale  gray  or  reddish-gray  in  color;  it  consists  of  a  covering 
of  ciliated  epithelium  and  a  connective-tissue  layer.  It  is  without  glands,  the 
glands  not  appearing  usually  until  the  third  or  fourth  year,  and  developing  very 
slowly  up  to  the  age  of  puberty. 

Menstruation. 

The  function  of  menstruation  involves  great  changes  in  the  mucosa  of  the 
body  of  the  uterus.  We  distinguish  three  periods :  (i)  tumefaction  of  the  mu- 
cosa, with  accompanying  structural  changes,  taking  five  days,  or,  according  to 
Hensen,  ten  days;  (2)  menstruation  proper,  about  four  days;  (3)  restoration  of 

316 


MENSTR  UA  TION.  317 

the  resting  mucosa,  about  seven  days.  The  times  given  are  approximative  only. 
The  whole  cycle  of  changes  covers  about  sixteen  days.  Since  the  monthly 
period  is  about  four  weeks,  the  period  of  rest,  as  thus  calculated,  is  only  about 
twelve  days. 

1.  Tumefaction.— A  few  days  before  the  menstrual  flow  the  mucosa  gradu- 
ally thickens;  the  surface  becomes  irregular;  the  openings  of  the  glands  lie  in 
depressions.     The  connective-tissue  cells  are  increased  in  number,  and  it  is  said 
by  some  authors  in  size,  but  the  increase  in  size  is  doubtful ;  the  number  of  round 
cells  increases ;  the  glands  expand  and  become  more  irregular  in  their  course ;  a 
short  time  before  hemorrhage  begins,  the  blood-vessels,  especially  the  capillaries 
and  veins,  become  greatly  distended.     We  must  assume  that  the  connective- 
tissue  cells  proliferate,  but  we  have  no  satisfactory  observations  upon  their  divi- 
sion.    It  was  formerly  asserted  that  the  menstrual  decidua  contains  decidual 
cells,  but  in  all  the  specimens  the  author  has  studied  there  were  none  present. 

2.  Menstruation. — When  the   changes   just   described  are  completed,  the 
decidua  menstrualis  is  fully  formed,  and  its  partial  disintegration  begins.     The 
process  commences  with  an  infiltration  of  blood  into  the  subepithelial  tissues; 
this  infiltration  has  hitherto  been  commonly  explained  as  due  to  the  rupture  of 
the  capillaries;  but  as  no  ruptures  at  this  period  have  been  observed,  we  may 
justly  regard  this  explanation  as  inadmissible,  and  account  for  the  infiltration  per 
diapedesin.     It  lasts  for  a  day  or  two,  and  is  apparently  the  immediate  cause  of 
a  very  rapid  molecular  disintegration  of  the  superficial  layers  of  the  mucosa, 
which  in  consequence  are  lost;  the  superficial  blood-vessels  are  now  exposed, 
and,  by  rupturing,  cause  the  well-known  hemorrhagia  of  menstruation.     By  the 
disappearance  of  its  upper  portion  the  mucosa  is  left  without  any  lining  epithe- 
lium and  is  very  much   (and  abruptly)  reduced  in  thickness.     Its  surface  is 
formed  by  connective  tissue  and  exposed  Wood-vessels. 

3.  Restoration  of  the  Mucosa. — At  the  close  of  menstruation  the  mucosa  is 
2  or  3  mm.  thick;  the  regeneration  of  the  lost  layers  begins  promptly  and  is  com- 
pleted in  a  variable  time,  probably  from  five  to  ten  days.     The  hyperemia  rapidly 
disappears;  the  extravasated  blood-corpuscles  are  partly  resorbed,  partly  cast 
off;  the  spindle-cell  network  grows  upward,  while  from  the  cylinder  epithelium 
of  the  glands  young  cells  grow  and  spread  up  and  out  so  as  to  produce  a  new 
epithelial  covering;  new  subepithelial  capillaries  appear.     The  details  of  these 
changes  are  imperfectly  known;  they  effect  the  return  of  the  mucosa  to  its 
resting-stage. 

Decidua  Menstrualis. — Specimens  from  the  first  day  of  menstruation  are  the 
most  instructive.  They  should  be  preserved  in  Zenker's  fluid;  sections  may  be 
made  perpendicular  to  the  decidual  surface  from  blocks  10  to  15  mm.  cube,  and 


318 


HUMAN  UTERUS  AND  F(ETAL  APPENDAGES. 


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THE  PREGNANT  UTERUS.  319 

stained  with  alum  hematoxylin  and  eosin.  The  use  of  Mallory's  triple  connec- 
tive-tissue stain  will  demonstrate  the  fibrillar  tissue  in  the  decidua  and  the  very 
large  amount  of  the  same  in  the  muscularis. 

The  accompanying  figure  is  from  a  uterus  in  active  menstruation.  The 
decidual  membrane  is  from  i.i  to  1.3  mm.  thick;  its  surface  is  irregularly  tume- 
fied ;  the  gland  openings  lie  for  the  most  part  in  the  depressions.  In  the  cavity 
of  the  uterus  there  was  a  small  blood-clot.  The  demarcation  between  the  de- 
cidua and  the  muscularis  is  sharp.  The  upper  fourth,  d,  of  the  decidua  is  broken 
down  and  very  much  disintegrated ;  its  cells  stain  less  readily  than  those  of  the 
deep  portion  of  the  membrane ;  the  tissue  is  divided  into  numerous  more  or  less 
separate  small  masses.  Some  of  the  blood-vessels  are  ruptured.  The  super- 
ficial epithelium,  ep,  is  loosened  everywhere ;  in  places  fragments  of  it  have  fallen 
off,  and  in  some  parts  it  is  gone  altogether ;  it  stains  readily  with  alum  hematox- 
ylin, differing  in  this  respect  from  the  underlying  connective  tissue.  The  deeper 
layer  of  the  decidua  is  dense  with  crowded  well-stained  cells,  which  lie  in  groups 
and  are  probably  proliferated  connective-tissue  cells.  They  have  small  oval  or 
elongated  darkly  stained  nuclei,  with  very  small  granular  protoplasmatic  bodies. 
There  is  no  indication  of  any  enlargement  of  the  cells,  such  as  occurs  in  the  pro- 
duction of  true  "  decidual"  cells.  There  are  very  few  leucocytes.  The  glands 
are  enlarged  somewhat,  and  are  lined  by  a  normal  cylinder  epithelium,  which 
offers  no  obvious  change  as  compared  with  that  of  the  glands  of  the  resting  uterus. 

The  Pregnant  Uterus:  the  Two  Stages. 

When  the  ovum  implants  itself  in  the  uterine  wall,  it  becomes  covered  by  a 
growth  of  the  mucous  membrane  or  decidua  which  we  know  as  the  decidua  re- 
flexa.  For  an  account  of  this  process  see  pages  118  to  120,  where  also  proper 
definitions  of  the  terms  decidua  reflexa,  serotina,  and  vera  are  given.  As  the 
ovum  increases  in  size  the  decidua  reflexa  also  increases,  and  gradually  becomes 
thinner  and  thinner,  until  it  ultimately  disappears.  The  exact  date  of  its  disap- 
pearance is  not  known;  it  falls  somewhere  within  the  fifth  month.  Accord- 
ingly, we  may  divide  the  period  of  pregnancy  into  two  phases  or  stages,  each 
comprising  about  half  of  the  whole  period.  During  the  first  stage  the  decidua 
reflexa  is  present,  during  the  second  stage  it  is  absent,  so  that  the  chorion  laeve 
comes  into  direct  contact  with  the  decidua  vera.  In  the  following  sections  a 
typical  uterus  of  the  first  and  second  stages  each  is  described. 

Human  Uterus  Three  Months  Pregnant. 

The  uterus  measures  about  3^  inches  in  transverse  diameter,  and  shows 
well-marked  venous  sinuses  on  its  external  surface.  It  should  be  opened  by  a 


320  HUMAN  UTERUS  AND  FCETAL  APPENDAGES. 

crucial  incision  on  the  anterior  side ;  its  walls  will  be  found  about  an  inch  or  more 
in  thickness;  it  contains  a  grayish-red  bag  (decidua  reflexa),  which  nearly  fills 
the  cavity  of  the  uterus  and  encloses  the  embryo,  so  that  upon  opening  the  womb 
we  do  not  encounter  the  foetus  directly.  The  inner  bag  has  a  smooth  surface, 
but  shows  a  few  small  pores;  it  is  without  blood-vessels  and  is  attached  to  the 
dorsal  wall  of  the  uterus.  The  inner  surface  of  the  uterus  shows  a  rich  network 
of  blood-vessels,  many  of  which  are  large,  irregular  sinuses.  The  uterine  walls 
consist  of  an  outer  muscular  layer,  and  an  inner  decidual  layer,  which  takes  up 
nearly  half  the  thickness  of  the  wall,  and  is  known  as  the  decidua  vera.  Com- 
parison with  the  seventh  month  uterus  shows  that  the  proportion  of  the  layers 
changes,  because  during  gestation  the  muscular  layer  increases  and  the  decidual 
layer  diminishes  in 'thickness.  The  inner  bag,  when  opened,  shows  the  large  cav- 
ity in  which  the  embryo  lies,  floating  in  amniotic  fluid.  The  bag  is  formed  by 
three  very  distinct  membranes,  of  which  the  outermost,  the  decidua  reflexa,  is 
opaque  and  the  thickest ;  the  two  inner  ones  are  thin  and  transparent ;  the  inner- 
most is  the  delicate  amnion;  the  middle  membrane  is  the  chorion,  and  is  quite 
distinct  from  both  the  amnion  and  reflexa ;  with  the  latter  it  is  connected  by  a 
number  of  small  branching  villi  scattered  at  some  distance  from  one  another  over 
the  surface;  the  villi  adhere  firmly  to  the  reflexa  by  their  tips.  The  embryo 
(Fig.  94)  resembles  a  child  in  its  general  appearance;  the  length  of  the  head 
and  rump  together  is  about  8  cm.,  and  the  head  is  approximately  of  equal  bulk 
to  the  rump.  The  umbilical  cord  is  from  5  to  7  mm.  in  diameter  and  usually  about 
1 2  cm.  long.  From  its  distal  end  the  blood-vessels  spread  out  over  the  placental 
area,  and  around  the  edge  of  the  area  rises  the  decidua  reflexa,  which  does  not 
extend  on  to  the  placenta.  Floating  in  the  amniotic  fluid  is  a  pear-shaped 
vesicle,  the  yolk-sac,  which  is  about  7  mm.  in  diameter;  it  has  a  fine  network  of 
blood-vessels  upon  its  surface,  and  is  connected  at  its  pointed  end  with  a  long 
slender  pedicle,  the  yolk-stalk,  which  runs  to  the  placental  end  of  the  umbilical 
cord,  there  enters  the  cord  itself,  and  runs  through  its  entire  length  to  its  attach- 
ment to  one  of  the  coils  of  the  intestine  of  the  embryo.*  Over  the  whole  of  the 
placental  area  the  chorion  gives  off  large  villous  trunks,  each  of  which  has  numer- 
ous branches,  with  ramifications  of  the  foetal  vessels;  the  villi  fill  a  space  about 
one  centimeter  wide  between  the  membrane  of  the  chorion  frondosum  and  the 
surface  of  the  uterine  decidua  serotina,  to  which  the  tips  of  some  of  the  villi  are 
attached.  With  care  the  villi  may  be  separated  from  the  decidua,  which  is  seen, 
when  it  is  thus  uncovered,  to  be  cavernous ;  the  caverns  are  rounded  in  form  and 

*  At  this  stage  a  large  part  of  the  yolk-stalk  within  the  umbilical  cord  has  degenerated  and  usually  disap- 
peared by  resorption. 


THE  PREGNANT  UTERUS.  321 

part  of  them  may  be  followed,  on  the  one  hand,  until  they  connect  with  the  blood 
sinuses  of  the  uterus,  and,  on  the  other,  until  they  open  into  the  intervillous 
spaces,  which  therefore  receive  a  direct  supply  of  blood  from  the  mother. 

The  principal  difference  to  be  noted  between  the  uterus  before  and  that 
after  the  fifth  month  in  the  relations  of  parts  is  the  presence  or  absence  of  the 
decidua  reflexa  as  a  distinct  membrane.  During  the  fourth  month  the  reflexa 
stretches  as  the  membranes  expand,  and  becomes  thinner  and  thinner  until  by 
the  end  of  the  fourth  month  it  is  as  delicate  and  transparent  as  the  chorion  and 
lies  close  against  the  decidua  vera. 

Human  Uterus  Seven  Months  Pregnant. 

If  we  examine  a  pregnant  uterus  at  any  time  during  the  sixth  to  ninth 
month  of  gestation,  we  find  essentially  the  same  relations  of  the  parts — the  most 
marked  difference  being  in  the  size  of  the  uterus,  which  increases  with  the  dura- 
tion of  gestation,  to  correspond  to  the  growth  of  the  foetus.  A  description  of  a 
uterus  seven  months  after  conception  will  suffice,  therefore,  for  our  present 
purpose. 

Such  a  uterus  is  a  large,  rounded  bag,  with  muscular  walls,  and  measures 
seven  or  eight  inches  in  diameter.  Examined  externally,  it  is  remarkable  espe- 
cially for  the  numerous  large  sinus-like  blood-vessels ;  its  surface  is  smooth ;  the 
texture  of  the  walls  is  firm  to  the  touch,  but  the  walls  yield  to  pressure,-  so  that 
the  position  of  the  child  can  be  felt.  As  the  placenta  is  generally  upon  the 
dorsal  side,  it  is  usual  to  open  the  uterus  by  a  crucial  incision  upon  the  ventral 
side.  The  walls  are  about  one-half  of  an  inch  thick,  sometimes  more,  sometimes 
less,  and  as  soon  as  they  are  cut  open  we  enter  at  once  into  the  cavity  of  the 
uterus  containing  the  foetus  and  nearly  a  pint  of  serous  liquid — the  amniotic 
fluid.  The  foetus  normally  lies  on  one  side,  has  the  head  bent  forward,  the  arms' 
crossed  over  the  chest,  the  thighs  drawn  against  the  abdomen,  and  the  legs 
crossed;  it  resembles  closely  the  child  at  birth,  but  is  smaller;  its  head  is,  rela- 
tively to  the  size  of  the  body,  larger;  the  abdomen  is  more  protuberant,  and  the 
limbs  proportionately  smaller.  The  inner  surface  of  the  uterus  is  smooth  and 
glistening;  if  it  is  touched  with  the  finger,  it  is  found  to  be  covered  by  a  thin  but 
rather  tough  membrane,  called  the  amnion,  which  is  only  loosely  attached.  Ex- 
amination of  the  uterine  wall,  where  it  has  been  cut  through,  shows  that  its 
thickness  is  formed  principally  by  the  muscular  layer,  which  is  made  up  by 
numerous  laminae  of  fibers,  between  which  are  the  large  and  crowded  blood 
sinuses,  similar  to  those  distinguishable  on  the  external  surface  of  the  uterus. 
About  one-fifth  or  less  of  the  wall  inside  the  muscularis  has  a  different  texture 
and  can  be  partly  peeled  off  as  two  distinct  membranes,  the  innermost  of  which 

21 


322  HUMAN  UTERUS  AND  FCETAL  APPENDAGES. 

is  the  amnion  already  mentioned,  and  the  outer  is  the  chorion  united  with  the 
decidua.  The  amnion  and  chorion  are  appendages  of  the  embryo;  the  decidua 
is  the  modified  mucous  membrane  of  the  uterus..  Let  us  return  to  the  embryo. 
From  its  abdomen  there  springs  a  long,  whitish  cord,  known  as  the  umbilical  cord; 
it  is  usually  from  about  one-third  to  one-half  an  inch  in  diameter  and  40  cm. 
long,  but  its  dimensions  are  extremely  variable;  it  always  shows  a  spiral  twist, 
and  contains  three  large  blood-vessels,  two  arteries  and  one  vein,  all  of  which 
can  be  distinguished  through  the  translucent  tissue.  The  distal  end  of  the  cord 
is  attached  to  the  wall  of  the  uterus,  usually  near  the  middle  of  the  dorsal  side  of 
the  organ.  It  is  easily  seen  that  the  blood-vessels  of  the  umbilical  cord  radiate 
out  from  its  end  over  the  surface  of  the  uterus  underneath  the  amnion,  branching 
as  they  go;  they  spread,  however,  only  over  a  circumscribed  area,  the  placental, 
where  the  wall  of  the  uterus  is  very  much  thickened.  A  vertical  section  through 
the  placental  area  shows  that  the  amnion  and  chorion  are  widely  separated  from 
the  decidua  and  muscularis  by  a  spongy  mass  soaked  with  maternal  blood.  This 
mass  consists  of  numerous  trees  of  tissue,  which  spring  with  comparatively  thick 
stems  from  the  chorion  and  branch  again  and  again.  In  these  stems  and 
branches  are  to  be  found  the  final  ramifications  of  the  vessels  of  the  umbilical 
cord;  the  trees  are  known  as  chorionic  or  placental  villi.  Some  of  their  end- 
twigs  are  very  closely  attached  to  the  surface  of  the  decidua.  In  the  center  of 
the  placental  area  the  villi  form  a  mass  about  three-fourths  of  an  inch  thick,  but 
toward  the  edge  of  the  area  the  mass  gradually  thins  out  until  at  the  very  edge 
the  chorion  and  decidua  come  into  immediate  contact.  The  mass  of  villi, 
together  with  the  overlying  portions  of  the  chorionic  and  amniotic  membranes 
and  the  underlying  portion  of  the  decidua,  constitutes  what  is  known  as  the 
placenta.  The  decidua  of  the  placental  area  is  called  the  decidua  serotina ;  the 
chorion  of  the  placenta  is  known  as  the  chorion  frondosum.  When  birth  takes 
place,  the  whole  placenta  is  expelled  after  the  delivery  of  the  child ;  the  placenta 
of  the  obstetrician  is  therefore  partly  of  foetal,  partly  of  maternal,  origin. 

Decidua  Vera  of  the  First  Stage  in  Section. 

Specimens  may  be  preserved  in  Zenker's  or  Tellyesnicky's  fluid,  or  they 
may  be  preserved  with  less  good  results  in  Miiller's  or  Parker's  fluid  or  in  picro- 
sulphuric  acid.  Sections  may  be  made  of  the  entire  wall  in  celloidin,  or,  if  it  is 
desired  to  get  thinner  sections,  in  paraffin,  in  which  case  it  is  advantageous  to 
remove  as  much  as  possible  of  the  muscular  coat  so  as  to  cut  only  the  decidual 
membrane. 

The  following  description  is  based  upon  a  uterus  one  month  pregnant. 
Figure  184  was  obtained  from  a  vertical  section  of  the  decidua,  by  drawing  the 


DECIDUA    VERA   OF  FIRST  STAGE. 


323 


outlines  of  the  glands  or  gland  spaces,  gl'  and  gl" ',  and  by  dotting  the  areas  occu- 
pied by  the  connective  tissue.  The  blood-vessels  are  indicated  by  double  out^ 
lines.  The  artery  shown  in  the  figure,  owing  to  its  spiral  course,  is  cut  repeatedly. 
The  figure  demonstrates  very  clearly  that  the  gland  cavities  are  so  arranged  that 
the  decidua  is  divided  into  an  upper  compact  layer  and  a  lower  cavernous  layer, 
the  difference  being  due  to  the  size  and  number  of  the  gland  cavities.  The 
amount  of  epithelium  to  be  observed  at  this  stage  varies  greatly.  It  is  some- 


Comp.  ( 


Art. 


Cav. 


Muse. 


FIG.  184. — VERTICAL  SECTION  OF  A  HUMAN  UTERUS 
(DECIDUA  VERA),  ONE  MONTH  PREGNANT. 

Comp,  Compact    layer.      Cav,  Cavernous    layer.     D, 
Gland-duct.       Art,    Spiral    artery.       Gl,  Spaces 
occupied  by  epithelial  glands.     Muse,  Muscularis. 
(For   clearness  all  the  glandular  epithelium  has. 
been  omitted  from  the  drawing.) 


FIG.  185. — HUMAN  UTERUS,  ONE  MONTH  PREG- 
NANT. SECTION  OF  GLAND  FROM  THE 
CAVERNOUS  LAYER,  WITH  THE  EPITHELIUM 
PARTLY  ADHERENT  TO  THE  WALLS.  X  445 
diams. 


times  wholly  absent  from  the  surface,  in  other  cases  absent  or  present  in  patches. 
In  the  glands  the  epithelium  has  undergone  many  modifications.  In  some  parts 
the  original  cylinder  epithelium  of  the  glands  is  well  preserved  in  patches,  and 
such  patches  of  epithelium  are  found  at  every  stage  until  after  delivery.  It  has 
been  observed  that  these  patches  serve  to  regenerate  the  epithelium  of  the  glands, 
and  by  spreading  from  the  glands  on  to  the  surface,  to  regenerate  also  the  epithe- 
lial covering  of  the  uterine  mucosa.  But  for  the  mostjmrt  the  glandular  epi- 


324 


HUMAN  UTERUS  AND  FCETAL   APPENDAGES. 


thelium  is  considerably  altered.  We  find  places  in  which  the  cells,  though 
attached  to  the  surrounding  connective  tissue,  are  separated  from  one  another 
by  small  fissures.  In  other  places  the  cells  are  a  little  larger  (Fig.  185),  each  for 
the  most  part  cleft  from  its  fellow,  and  some  of  them  loosened  from  the  wall  and 
lying  free  in  the  cavity.  Apparently  the  cells  which  are  thus  freed  become 
swollen,  probably  by  imbibition,  both  the  protoplasm  and  the  nuclei  becoming 
enlarged  (Fig.  186).  The  cells  lie  separately  and  almost  completely  fill  the  gland 
cavity.  They  are  no  longer  cylindrical  in  shape,  but  irregular.  Their  proto- 
plasm is  finely  granular  and  stains  rather  lightly.  The  nuclei  are  rounded,  gran- 
ular, and  with  sharp  outlines.  In  somewhat  older  stages  one  finds  the  cells 
replaced  by  a  granular  material.  The  obvious  interpretation  of  the  appearances 


FIG.  186. — HUMAN  UTERUS,  ONE  MONTH  PREGNANT.     SECTION  OF  A  GLAND  FROM  THE  CAVERNOUS  LAYER 
WITH  THE  EPITHELIUM  LOOSENED  FROM  THE  WALLS.     THE   EPITHELIAL  CELLS  ARE  SWOLLEN. 


described  is  that  the  glandular  epithelium  is  breaking  down  and  disintegrating, 
or,  in  other  words,  passing  through  a  special  form  of  degeneration  which  is 
highly  characteristic.  In  later  stages  some  of  the  broken-down  material  forms 
hyaloid  rounded  concretions,  which,  owing  to  their  deep  staining,  are  somewhat 
conspicuous. 

The  formation  of  decidual  cells  has  already  begun  in  the  upper  or  compact 
layer  (Fig.  187).  They  are  modified  connective-tissue  cells,  which  have  grown 
in  size  and  altered  their  structure.  Their  bodies  stain  deeply  with  eosin;  the 
nuclei  are  round,  oval,  slightly  irregular  in  shape,,  coarsely  granular,  and  sharp 
in  outline.  The  cells  themselves,  though  irregular  and  variable  in  shape,  are  all 
more  or  less  provided  with  processes  running  off  in  various  directions.  Scat- 


DECIDUA  RE  FLEX  A   OF  FIRST  STAGE. 


325 


tered  between  the  cells  are  many  sections  of  the  processes.  Occasionally  it  may 
be  seen  that  two  cells  are  connected.  Later  on  the  decidual  cells  acquire 
smoother  and  more  rounded  outlines,  and  appear  to  lose  altogether  their  connec- 
tions with  one  another.  In  the  cavernous  layer  there  are  no  decidual  cells. 


FIG.  187. — UTERUS  ONE  MONTH  PREGNANT  ;  PORTION  OF  THE  COMPACT   LAYER  OF  THE   DECIDUA  SEEN 

IN  VERTICAL  SECTION. 
Coagl,  Coagulum  upon  the  surface.     d,d',  Decidual  cells.     X  445  diams. 


Decidua  Reflexa  of  the  First  Stage. 

The  decidua  reflexa  may  be  preserved  in  Zenker's  fluid,  Parker's  fluid,  or 
picro-sulphuric  acid.  It  should  be  hardened  with  the  portions  of  the  chorion 
and  chorionic  villi  adherent  to  it.  It  may  be  imbedded  in  celloidin  and  the 
sections  stained  with  alum  hematoxylin  and  eosin,  with  Beale's  carmine,  or  with 
a  so-called  fibrin  stain. 

As  stated  above  (page  319),  the  presence  of  the  decidua  reflexa  distinguishes 
the  first  stage  of  pregnancy  from  the  second,  in  which  the  reflexa  is  absent,  hav- 
ing disappeared  by  degeneration  and  absorption.  To  observe  this  process  of  the 


326 


HUMAN  UTERUS  AND  FCETAL  APPENDAGES. 


disappearance  of  the  reflexa,  membranes  from  the  second  and  third  months 
should  be  examined. 

Section  of  Decidua  Reflexa  of  Two  Months. — At  this  time  the  reflexa  starts 
from  the  edge  of  the  placental  area  as  a  membrane  of  considerable  thickness,  but 
it  rapidly  thins  out,  the  very  thinnest  point  being  opposite  the  placenta.  Ex- 
amination of  sections  shows  that  the  entire  reflexa  is  undergoing  degeneration 

which  is  found  to  be  more  advanced  the 
more  remote  the  part  examined  is  from 
the  placenta.  The  chorion  laeve  lies 
very  near  the  reflexa,  being  separated 
only  by  the  villi,  which  are  already  very 
much  altered  by  degeneration.  In  the 
region  half-way  between  the  base  and 
the  apex  of  the  reflexa  the  tissue  (Fig. 
1 88)  shows  only  vague  traces  of  its 
original  structure.  Only  here  and  there 
can  a  distinct  cell  with  its  nucleus  be 
made  out.  Most  of  the  cells  have 
broken  down  and  fused  into  irregular 
hyaline  masses  without  organization. 
Ramifying  through  the  fused  detritus 
appear  strands  and  lines,  which  are 
more  darkly  stained  by  both  carmine 
and  hematoxylin.  On  account  of  their 
fibrous  appearance,  these  strands  are 
often  spoken  of  as  fibrin,  although  they 
are  presumably  not  the  same  as  the 
true  fibrin  from  the  blood.  The  fibrin 
is  much  more  developed  upQn  the  inner 
or  chorionic  side  than  upon  the  outer 
side  of  the  reflexa.  On  the  inner  side  it 
forms  a  dense  network,  which  fuses  with 
the  degenerated  ectoderm  of  the  cho- 
rionic villi  wherever  the  villi  are  in  con- 
tact with  the  decidua.  It  also  ramifies  nearly  half-way  through  the  decidua, 
the  ramifications  being  followed  easily,  owing  to  the  dark  staining  of  the  sub- 
stance. Over  the  outside  of  the  decidua  the  fibrin  forms  a  much  thinner  layer 
or  may  be  only  indistinctly  formed. 

In  a  decidua  reflexa  of  three  months  the  conditions  are  essentially  the  same, 


FIG.  188.— SECTION  OF  HUMAN  DECIDUA  REFLEXA 
AT  Two  MONTHS. 


DECIDUA  VERA  AND  CH ORION L^E VE  OF  THE  SECOND  STAGE.  327 

except  that  the  degeneration  is  further  advanced  and  the  membrane  thinner. 
Traces  of  cellular  structure  are  still  more  vague  and  the  fibrin  is  more  developed. 
In  all  parts  of  the  membrane  there  appear  leucocytes  which  are  particularly 
numerous  and  conspicuous  in  the  neighorhood  of  the  placenta.  It  is  natural 
to  assume  that  they  are  concerned  in  the  resorption  of  the  reflexa.  There  is 
an  inner  thicker  layer  of  fibrin  and  a  thinner  outer  layer,  which  is  now  always 
present  and  distinct.  Between  these  two  layers  is  a  stratum  in  which  the  re- 
mains of  the  cells  may  be  seen.  Occasionally  there  is  an  appearance  which  sug- 
gests surviving  decidual  cells,  and,  indeed,  in  sections  taken  from  parts  close  to 
the  placenta  true  decidual  cells  may  be  identified. 

The  origin  of  the  chorion  laeve  and  the  disappearance  of  its  villi  have  been 
described  (page  121).  The  sections  of  the  decidua  reflexa  will  serve  also  to 
enable  the  student  to  see  some  of  the  phases  of  the  degeneration  of  the.  villi. 
They  are  very  much  altered.  Their  ectoderm  undergoes  a  hypertrophic  degen- 
eration and  becomes  hyaline  tissue,  which  stains  darkly.  The  degenerated 
ectoderm  of  adjacent  villi  fuses  more  or  less  extensively.  The  mesoderm  of  the 
villi  shows  a  partial  loss  of  its  primitive  cellular  organization. 

Decidua  Vera  and  Chorion  Laeve  of  the  Second  Stage. 

Pieces  of  the  decidua  vera  of  from  six  to  nine  months  with  the  chorion  and 
amnion  carefully  preserved  in  situ  may  be  hardened  in  Miiller's  or  Telly esnicky's 
fluid.  Blocks  half  an  inch  or  less  in  size  may  be  imbedded  in  celloidin  and  sec- 
tions made  perpendicularly  to  the  surface,  stained  with  alum  hematoxylin  and 
eosin,  or  with  Heidenhain's  hematoxylin  and  orange  G,  or  with  picro-carmine. 

The  decidua  reflexa  having  been  resorbed,  the  chorion  (Fig.  189,  Cho)  has 
come  into  contact  with  the  surface  of  the  uterus,  and  the  chorionic  epithelium,  c, 
is  closely  adherent  to  the  surface  of  the  decidua,  from  which  the  original  epithe- 
lium has  completely  disappeared.  The  amnion  is  loosely  connected  with  the 
chorion  by  ,a  few  strands  or  threads,  which  are  represented  in  the  figure  and 
the  nature  of  which  is  not  known.  Both  the  amnion,  Am,  and  the  chorion, 
Cho,  being  developed  from  the  original  somatopleure  (compare  page  78), 
consist  of  a  mesodermic  and  an  ectodermal  layer.  The  ectoderm  of  the 
amnion  is  a  single  layer  of  cuboidal  cells  placed  on  the  side  of  the 
membrane  toward  the  embryo  and  away  from  the  uterus.  The  ecto- 
derm, c,  of  the  chorion,  on  the  contrary,  is  next  the  uterus.  Hence  it  will 
be  noticed  that  the  mesodermic  layers  of  the  amnion  and  chorion  are 
adjacent.  Both  membranes  are  quite  thin.  The  decidua  is  a  relatively 
voluminous  membrane  containing  blood-vessels,  u,  which  for  the  sake  of  dis- 
tinctness have  been  filled  in  with  black  in  the  drawing.  It  also  contains  a  series 


328 


HUMAN  UTERUS  AND  FCETAL  AP.PENDAGES. 


of  elongated  spaces,  which  represent  sections  of  the  glands.  These  spaces,  gl, 
are  present  only  in  the  inferior  half  of  the  decidua.  Owing  to  their  absence 
from  the  superior  half,  that  portion  has  a  more  compact  structure,  and  is,  there- 
fore, designated  as  the  compact  layer;  the  lower  portion,  being  broken  up  and 
made  loose  in  texture  by  the  somewhat  numerous  gland  cavities,  is  called  the 
cavernous  layer,  the  caverns,  of  course,  corresponding  to  the  gland  spaces.  The 
gland  spaces  are  now  very  much  stretched  out,  a  condition  which  results  simply 


FIG.  189. — HUMAN   UTERUS   ABOUT   SEVEN   MONTHS    PREGNANT.     VERTICAL   SECTION  OF   THE  DECIDUA 

VERA  WITH  THE  FCETAL  MEMBRANES  IN  SITU. 

Am,  Amnion.      Cho,  Chorion.     c,  Chorionic  epithelium,     z/,  Blood-vessel,    gl,  Glands,     muse,  Muscularis. 

X  40  diams. 

fromjthe  general  expansion  of  the  uterus  during  pregnancy.  In  the  gland  spaces 
appear  patches  of  epithelium  still  intact,  and  in  the  cavities  themselves 
isolated  cells  in  various  phases  of  degeneration  and  disintegration,  similar  to  the 
phases  which  may  be  observed  in  the  decidua  vera  of  one  month ;  .but  the  degen- 
eration is,  on  the  whole,  considerably  more  advanced  than  in  the  early  stage. 
Around  some  of  the  larger  blood-vessels  there  is  connective  tissue  but  slightly 


THE  PL  A  CENTA  IN  SITU.  329 

modified,  and  the  original  structure  of  the  mucous  membrane  is  more  or  less,  but 
not  perfectly,  preserved  in  the  deep  portion  of  the  decidua.  But  the  majority 
of  the  cells,  especially  in  the  compact  layer,  have  grown  in  size  and  become  trans- 
' formed  into  true  decidual  cells.  In  the  ectoderm  of  the  chorion,  c,  the  cells  lie 
two  or  tjiree  deep.  They  have  distinct  walls,  a  very  coarsely  granular  proto- 
plasm, and  nuclei  which  stain  darkly.  By  these  characteristics  they  are  easily 
distinguished  from  the  neighboring  decidual  cells,  to  which,  however,  they  offer 
a  slight  superficial  resemblance.* 

The  Placenta  in  Situ. 

The  placenta  in  its  natural  position  in  the  uterus  follows  the  curvature  of  the 
uterine  walls,  hence  its  free  or  amniotic  surface  is  slightly  concave.  Its  decidual 
surface  is  strongly  convex.  It  is  thickest  in  the  center  and  thins  out  gradually 
toward  its  edge.  The  uterus  should  be  obtained  in  the  freshest  possible  condi- 
tion and  be  opened  by  a  crucial  incision  on  the  ventral  side.  The  embryo  should 
then  be  removed,  the  umbilical  cord  cut  through,  care  being  taken  to  bring  as 
little  pressure  as  possible  on  the  uterus  or  the  placenta.  The  whole  organ  is  then 
placed  in  the  preseryative,  which  should  be  either  Telly esnicky's  or  Muller's 
fluid.  In  view  of  the  large  size  of  the  organ,  it  is  very  necessary  to  use  large  quan- 
tities of  the  preserving  fluid,  and  this  fluid  must  be  changed  several  times  in  order 
to  insure  good  histological  preservation.  When  the  hardening  is  completed, 
columns  about  one-half  inch  square  may  be  cut  out  so  as  to  pass  vertically 
from  the  inner  to  the  outer  surface  of  the  placenta,  preserving  the  amniotic  and 
chorionic  membranes  in  place.  The  blocks  are  to  be  imbedded  in  celloidin  and 
ought  to  remain  at  least  three  days  in  thin  and  three  days  in  thick  celloidin,  so  as 
to  insure  a  thorough  penetration  of  the  imbedding  material  into  the  intervillous 
spaces.  Make  the  sections  so  that  they  pass  vertically  through  the  placenta. 
Stain  with  hematoxylin  and  eosin  or  with  picro-carmine. 

Placenta  at  Seven  Months. — A  section  made  according  to  the  method  just 
described  is  represented  in  figure  190.  The  thin  amnion,  Am,  covers  the  upper 
(or  inner)  surface  of  the  chorionic  membrane,  Cho.  This  membrane  is  separated 
from  the  decidua,  D,  by  a  dense  forest  of  villi,  of  which  innumerable  sections 
appear.  In  younger  placentas  the  distance  between  the  chorion  and  the  de- 
cidua is  considerably  less,  and  the  number  of  sections  of  villi  is  smaller,  but  the 
average  size  of  those  sections  larger.  In  the  present  specimen  the  distance  be- 
tween the  chorion  and  the  decidua  is  nearly  twice  as  great  as  the  diameter  of  the 

*  It  should  perhaps  be  noted  that  in  some  comparatively  recent  text-books  the  chorionic  ectoderm  has  been 
described  as  the  decidua  reflexa,  an  error  which  is  much  to  be  regretted.  , 


V6. 


FIG.  190. — HUMAN  PLACENTA  IN  SITU,  ABOUT  SEVEN  MONTHS.    VERTICAL  SECTION. 

Am,  Amnion.     Cho,  Chorion.     Vi,  Villous  trunk,    vi,  Sections  of  villi  in  the  substance  of  the  placenta.     Df,D/f, 

Decidua  serotina.     Me,  Muscularis.      Ve,  Uterine  artery,  opening  into  the  placenta  ;  the  foetal  blood-vessels 

are  drawn  black ;  the  maternal  blood-vessels  white  ;  the  chorionic  tissue  is  stippled,  except  the  canalized 

.  fibrin,  which  is  shaded  by  lines.     The  remnants  of  the  gland  cavities  in  the  decidua  are  stippled  dark. 

X  6  diams. 

330 


THE  PLACENTA  IN  SITU. 


331 


muscular  coat,  Me,  of  the  uterus.  The  ends  of  some  of  the  villi  touch,  and  are 
imbedded  in,  the  decidual  tissue.  Their  imbedded  ends  are  without  covering 
epithelium,  but  their  connective  tissue  is  immediately  surrounded  by  hyaline 
substance  which  is  probably  degenerated  epithelium.  The  decidua  serotina  is 
plainly  divided  into  an  upper  compact,  Df,  and  a  lower  cavernous  layer,  D" .  The 
section  figured  passes  through  an  arterial  vessel,  ve,  which  makes  an  abrupt  turn 
so  as  to  discharge  its  blood  into  the  intervillous  spaces. 

The  histological  structure  of  all  the  parts  should  be  carefully  studied.  (As 
regards  the  structure  of  the  amnion,  see  page  349.) 

The  chorion  consists  of  two  layers,  the  outer  ectodermic  and  inner  meso- 
dermic.  Over  the  chorionic  membrane  proper  the  ectoderm  offers  a  great  variety 


Am. 


FIG.  191. — HUMAN  PI.ACENTAL  CHORION  AND  AMNION  OF  THE  FIFTH  MONTH. 

Ep,  Amniotic  epithelium.     Am,  Amnion.     Str,  Stroma.     Fib,  Fibrillar  layer.     Fbr,  Fibrin  layer,     c,  Chorionic 
cellular  layer  of  ectoderm.      Vi,  Chorionic  villi.     X  71  diams. 

of  appearances.  In  some  places  it  may  be  seen  to  have  still  its  primitive  organi- 
zation, a  single  inner  layer  of  distinct  cells  and  an  outer  syncytial  layer,  more  or 
less  similar  to  those  represented  in  figure  199.  For  the  most  part,  however,  the 
chorionic  ectoderm  has  been  considerably  modified  from  its  primitive  condition. 
The  inner  or  cellular  layer  exhibits  irregular,  thickened  patches,  which  present 
every  possible  degree  of  variation  as  to  their  size.  A  cell  patch  from  a  somewhat 
younger  stage  is  represented  in  figure  191  as  seen  with  a  low  magnification,  and 
another  patch  of  the  age  we  are  studying  is  represented  in  figure  192.  The 
patches  vary  in  appearance ;  the  cells  are  more  distinct  in  the  small  patches,  less 


332 


HUMAN  UTERUS  AND  FCETAL  APPENDAGES. 


so  in  the  large  patches,  in  which  there  are  often  parts  more  or  less  degenerated. 
The  cell-bodies  stain  lightly;  their  nuclei  are  granular, -not  very  sharply  denned, 
and  variable  in  size  and  shape.  The  cellular  layer  is  always  sharply  defined 
against  the  mesoderm.  Toward  the  outside  the  patches  offer  varying  relations. 
In  some  cases  a  part  of  a  cell  patch  may  form  the  whole  thickness  of  the  ecto- 
derm, as  shown  in  figure  191,  or  the  whole  of  a  cell  patch  may  do  so.  More 
commonly,  however,  the  cellular  patch  is  covered  more  or  less  completely  by  a 


mes 


FIG.  192. — HUMAN  CHORION  OF  SEVEN  MONTHS'  PLACENTA. 
c,  Cellular  layer,    fb,  Fibrin  layer,     ep,  Remnants  of  epithelial  layer,     mes,  Mesoderm.      X  445  diams. 

special  substance,  which  is  termed  canalized  fibrin,  and  which  is  believed  to 
represent  the  original  outer  syncytial  layer  in  a  degenerated  condition.  The 
fibrin  is  a  constant,  normal,  and  very  remarkable  constituent  of  the  placenta. 
Its  formation  seems  to  begin  always  in  the  outer  or  syncytial  layer  of  the  cho- 
rionic  ectoderm,  but  it  may  also  spread  into  the  cellular  layer,  which  then  be- 
comes replaced  by  fibrin,  so  that  this  last  alone  represents  the  ectoderm  of  the 


THE  PL  A  CENTA  IN  SITU.  333 

cliorion.  The  fibrin  layer  consists  of  a  very  refringent  substance  permeated 
by  numerous  channels  (Fig.  192,  /&).  The  substance  has  a  violent  affinity  for 
carmine  and  hematoxylin,  and  is  always  the  most  deeply  colored  part  of  a  section 
thus  stained.  The  channels  tend  to  run  more  or  less  parallel,  to  the  surface  of 
the  chorion,  and  are  connected  by  numerous  short'cross-channels.  Some  of  the 
channels  contain  cells  or  nuclei.  The  appearances,  however,  are  very  variable; 
the  fibrin  often  sends  long  outshoots  into  the  cellular  layers.  To  summarize,  we 
may  say  that  the  ectoderm  of  the  chorionic  membrane  undergoes  patchwise 
manifold  changes.  It  exists  in  three  general  forms :  the  nucleated  protoplasm 
or  syncytium,  the  cellular  layer,  and  the  canalized  fibrin.  A  patch  of  the  ecto- 
derm may  consist  of  any  one  of  these  modifications  or  any  two,  or  of  all  three. 
But  they  have  fixed  relative  positions,  for  when  the  syncytium  is  present,  it 
always  covers  the  free  surface  of  the  chorion;  when  the  cellular  layer  is  present, 
it  always  lies  next  the  mesoderm ;  and  when  all  three  forms  are  present  over  the 
same  part,  the  fibrin  is  always  the  middle  stratum. 

The  mesoderm  of  the  chorion  in  early  stages  has  a  homogeneous  matrix, 
which  about  the  ninth  week  begins  to  change  its  appearance.  In  the  frondosum, 
in  our  specimen,  the  matrix  has  acquired  a  distinctly  fibrous  structure.  Usually 
the  production  of  fibers  is  much  greater  in  the  immediate  neighborhood  of  the 
ectoderm,  and  this  may  go  so  far  as  to  mark  out  a  more  or  less  distinct  subecto- 
dermal  fibrillar  layer  (Fig.  191,  Fib}.  There  appears  to  be  no  mesothelial  layer 
upon  the  chorion  at  this  stage,  but  it  seems  possible  that  its  presence  might  be 
revealed  by  the  application  of  proper  special  methods. 

In  the  mlli  the  ectoderm  differs  from  that  of  the  chorionic  membrane  in 
several  respects:  (i)  The  cellular  layer  after  the  first  month  becomes  less  and 
less  conspicuous,  and  after  the  fourth  month  is  present  only  in  a  few  isolated 
patches,  which  have  been  termed  the  cell-knots.  (2)  For  the  most  part  the 
villi  remain  covered  by  the  syncytial  layer,  which  in  many  places  is  thickened. 
In  later  stages  these  thickenings  are  small  and  numerous,  constituting  the  so- 
called  proliferation  islands  with  many  nuclei.  Many  of  the  little  thickenings 
appear  in  sections  of  the  villi,  and  here  and  there  are  converted  into  canalized 
fibrin.  (3)  The  proliferation  islands  are  converted  into  canalized  fibrin  and 
at  the  same  time  grow  and  fuse,  forming  larger  patches,  particularly  on  the 
larger  stems.  In  this  manner  are  produced  the  large  areas  and  columns  of 
fibrin  such  as  appear  in  our  section.  (4)  Over  the  tips  of  the  villi,  where  they 
are  imbedded  in  the  decidua  serotina,  the  epithelium  apparently  degenerates 
and  becomes  hyaline  tissue,  but  without  canalization.  The  mesoderm  exists 
in  two  principal  forms,  adenoid  tissue  and  fibrillar  tissue  around  the  blood- 
vessels. The  adenoid  tissue  (Fig.  193)  may  be  considered  as  the  proper  tissue  of 


334 


HUMAN  UTERUS  AND  FCETAL  APPENDAGES. 


the  villus.  It  consists  of  a  network  of  protoplasmic  threads,  which  start  from 
nucleated  masses.  There  are  many  large  meshes,  which  are  partly  occupied  by 
the  very  large,  coarsely  granular,  wandering  cells,  /,  /.  The  wandering  cells 
generally  are  widely  scattered,  but  sometimes  are  present  in  large  numbers. 
They  are  usually  interpreted  as  foetal  leucocytes.  They  differ,  however,  by 
their  large  size  and  appearance  strikingly  from  the  leucocytes  of  the  adult.  We 
have  no  knowledge  of  their  history  or  functions.  About  the  capillary  blood- 
vessels, v,  the  network  is  more  finely  spun.  Around  the  larger  blood-vessels  the 
mesoderm  has  a  distinct  intercellular  substance  with  a  tendency  to  fibrillar 
differentiation  in  quite  a  wide  zone  around  the  blood-vessels.  In  this  zone  the 
cells  become  elongated  or  irregularly  fusiform.  Around  the  larger  vessels  the 


FIG.  193. — ADENOID  TISSUE  FROM  A  VILLUS  OF  A  HUMAN  PLACENTA  OF  FOUR  MONTHS. 

/,/,/,  Wandering  cells,    v,  z>,  Capillary  blood-  vessels,     d,  Finer  meshwork  surrounding  a  capillary.     X  352diams. 

i 

cells  are  grouped  in  laminae,  and  apparently  are  contractile,  so  that  they  must  be 
looked  upon  as  an  imperfectly  differentiated  form  of  smooth  muscular  tissue. 

Decidua  Serotina  at  Seven  Months. 

Specimens  may  be  treated  as  described  for  the  placenta  in  situ  (page  329). 
If,  however,  the  best  results  are  desired,  the  whole  of  the  uterus  should  be  cut 
through  and  the  placenta  divided  into  smaller  pieces  from  i  to  2  cm.  in  diameter, 
so  as  to  allow  a  freer  penetration  of  the  preserving  fluid.  Either  Zenker's  or 
Tellyesnicky's  fluid  is  recommended.  In  a  normal  uterus  about  seven  months 
pregnant  we  find  the  following  relations:  The  serotina  is  about  1.5  mm.  thick, 
and  contains  an  enormous  number  of  decidual  cells  (Fig.  194);  the  cavernous, 


DECIDUA  SEROTINA  AT  SEVEN  MONTHS. 


335 


D' ',  and  compact  layers,  D",  are  very  clearly  separated;  the  mucosa  is  sharply 
marked  off  from  the  muscularis,  although  scattered  decidual  cells  have  pene- 
trated between  the  muscular  fibers.  The  muscularis  is  about  10  mm.  thick  and 
is  characterized  by  the  presence  of  quite  large  and  numerous  venous  thrombi, 
especially  in  the  part  toward  the  decidua.  The  decidua  itself  contains  few  blood- 
vessels. Upon  the  surface  of  the  decidua  can  be  distinguished  a  special  layer  of 
denser  decidual  tissue,  which  in  many  places  is  interrupted  by  the  ends  of  the 
chorionic  villi  which  have  penetrated  it,  as  is  well  shown  in  the  accompanying 


Vi 


me 


FIG.  194. — THE  HUMAN  DECIDUA  SEROTINA  AT  SEVEN  MONTHS.     THE  SECTION   is    TAKEN   FROM   NEAR 

THE  MARGIN  OF  THE  PLACENTA. 

Vi,  Chorionic  villi ;  the  intervillous  spaces  were  filled  with,  maternal  blood,  which  is   not  represented  in  the 
figure.     D' ',  Cavernous  layer  of  the  decidua.     Z>/x,  Compact  layer  of  the  decidua.     me,  Muscularis. 


figure.  The  gland  cavities  of  the  spongy  layer,  D',  are  long  and  slit-like;  they 
are  filled  for  the  most  part  with  fine  granular  matter,  which  stains  light  blue  with 
hematoxylin ;  they  also  contain  a  little  blood,  and  sometimes  a  few  decidual  cells. 
There  also  occur  in  them  hyaloid  concretions, — oval  bodies  several  times  larger 
than  any  of  the  decidual  cells,  and  presenting  a  vacuolated  appearance.  In 
uteri  over  two  months  pregnant  they  are  probably  ^n variably  present.  In  many 
places  the  glandular  epithelium  is  perfectly  distinct;  its  cells  vary  greatly  in 


336 


HUMAN  UTERUS  AND  FCETAL  APPENDAGES. 


appearance,  neighbors  being  often  quite  dissimilar;  nearly  all  are  cuboidal,  but 
some  are  flattened  out;  of  the  former,  a  number  are  small  with  darkly  stained 
nuclei,  but  the  majority  of  the  cells  are  enlarged,  with  greatly  enlarged  hyaline, 
very  refringent  nuclei.  There  are  also  in  many  of  the  gland  spaces  isolated 
enlarged  cells,  which  have  detached  themselves  from  the  wall,  and  in  some  cases 
the  detached  cells  nearly  fill  the  gland  cavity,  very  much  as  in  figure  186. 

The  decidual  cells  of  the  cavernous  layer  (Fig.  194,  D'}  are  smaller  and  more 
crowded  than  most  of  those  of  the  compact  layer.  The  largest  cells  are  scat- 
tered through  the  compact  layer,  but  are  most  numerous  toward  the  surface. 
They  extend  around  the  margin  of  the  placenta  and  have  penetrated  the  chorion , 

in  the  cellular  layer  of  which  they  are 
very  numerous ;  the  immigration  im- 
parts to  the  chorionic  layer  in  ques- 
tion somewhat  the  appearance  of  a 
decidual  membrane.  Misled  by  this 
peculiarity,  some  authors  have  held 
this  layer  to  be  maternal  in  origin, 
and  accordingly  have  described  it  as 
a  "  decidua  subchorialis."  The  de- 
cidual cells  exhibit  great  variety  in 
their  features  (Fig.  195).  They  are 
nearly  all  oval  discs,  so  that  their 
outlines  differ  according  as  they  are 
seen  lying  in  the  tissue  turned  one 
way  or  another ;  they  vary  greatly  in 
size;  the  larger  they  are,  the  more 
nuclei  they  contain;  the  nuclei  are 
usually  more  or  less  elongated;  the 

contents  of  the  cell  granular.  Some  of  the  cells  present  another  type,  c;  these 
are  more  nearly  round,  are  clear  and  transparent;  the  nucleus  is  round,  stains 
lightly,  and  contains  relatively  few  and  small  chromatin  granules ;  such  cells  are 
most  numerous  about  the  placental  margin. 

The  Human  Placenta. 

Specimens  of  the  fresh  normal  human  placenta  can  be  obtained  without 
difficulty  from  maternity  hospitals.  The  specimen  should  be  thoroughly  ex- 
amined in  the  fresh  state  by  the  student  and  all  the  points  in  the  description 
below  verified  by  him.  To  intake  an  injected  specimen  either  the  starch  injection 
mass  or  the  colored  gelatin  mass  may  be  used  accordingly  as  it  is  desired  to 


FIG.   195. — DECIDUAL    CELLS    FROM   THE    SECTION 

REPRESENTED  IN  FIG.   194. 

<r,  Multinucleate  cell ;  at  a  seven  blood-corpuscles  have 
been  drawn  in  to  scale  as  a  measure  of  size. 


THE  HUMAN  PLACENTA.  337 

demonstrate  only  the  coarser  or  all  the  branches  of  the  vessels.  The  injection 
should  be  made  through  one  of  the  arteries  of  the  umbilical  cord.  As  there  is 
almost  invariably  a  cross-anastomosis  between  the  two  arteries  close  to  the 
placenta,  it  is  sufficient  to  inject  one  of  them  in  order  to  fill  the  entire  system  of 
vessels.  The  starch  mass  may  be  injected  in  the  cold  specimen.  If  the  gelatin 
mass  is  used,  the  specimen  must  be  submerged  in  warm  water  until  it  is  suffi- 
ciently heated  to  keep  the  gelatin  mass  melted  during  the  process  of  injection. 
After  the  gelatin  injection  is  completed,  the  placenta  may  be  preserved  in  70  per 
cent,  alcohol,  to  every  100  c.c.  of  which  2  c.c.  of  hydrochloric  acid  have  been 
added.  After  twenty-four  hours  the  acidulated  alcohol  may  be  replaced  by 
fresh  alcohol  of  70  per  cent., which  should  be  again  changed  after  another  twenty- 
four  hours.  Specimens  will  then  keep  indefinitely.  Such  specimens  may  be 
used  either  for  sections  of  the  placenta  to  be  made  from  pieces  imbedded  in  cel- 
loidin,  or  for  the  study  of  isolated  fragments  of  the  villi,  which  are  pulled  out  of 
the  placenta  by  forceps. 

The  human  placenta  is  a  disc  of  tissue  to  which  the  umbilical  cord  of  the 
child  is  attached  by  its  distal  end.  As  a  result  of  normal  labor  the  amnion 
and  chorion,  by  which  the  foetus  in  utero  is  surrounded,  are  ruptured;  the  child 
is  then  expelled,  but  by  means  of  the  long  umbilical  cord  remains  attached  to  the 
uterus;  after  an  interval  the  placenta,  with  which  the  cord  retains  its  connection, 
is  loosened  from  the  uterine  wall  and  expelled,  together  with  the  foetal  envelopes 
and  portions  of  the  decidual  membranes  (uterine  mucosa)  of  the  mother;  the 
parts  thus  thrown  off  secondarily  constitute  the  so-called  after-birth  of  obstetri- 
cians. 

The  placenta  at  full  term,  as  thus  obtained  by  natural  expulsion,  is  a  moist 
mass,  containing  a  great  deal  of  blood,  spongy  in  texture,  about  seven  inches  in^ 
diameter,  but  very  variable  in  size,  being  roughly  proportionate  to  the  bulk  of 
the  child ;  usually  oval,  sometimes  round,  but  not  infrequently  irregular  in  shape. 
One  surface  is  smooth  and  covered  by  a  pellucid  membrane  (the  amnion),  and 
reddish-gray  in  color ;  to  this  surface  the  umbilical  cord  is  attached,  and  it  shows 
the  arteries  and  veins  branching  out  irregularly  from  the  cord  over  the  surface  of 
the  placenta  (Fig.  196).  The  opposite  surface  is  rough,  lacerated,  and  usually 
covered  irregularly  with  more  or  less  blood,  which  is  often  dark  and  clotted. 
When  the  blood  is  removed,  the  surface  is  seen  to  be  crossed  by  a  system  of 
grooves  which  divide  the  placental  tissue  into  irregular  areas,  each  perhaps  an 
inch  or  so  in  diameter;  these  areas  are  called  cotyledons.  The  placenta  is  about 
25  or  30  mm.  thick,  but  thins  out  rapidly  at  the  edges,  and  its  tissue  passes  over 
from  the  margin  of  the  placenta. 

When  in  situ,  the  placenta  is  fastened  to  the  walls  of  the  uterus  by  its  rough 


338 


HUMAN  UTERUS  AND  F(ETAL  APPENDAGES. 


or  cotyledonary  surface;  its  smooth,  amniotic  surface  faces  the  cavity  in  which 
the  foetus  lies. 

A  more  detailed  examination  of  the  gross  appearance  of  a  placenta  dis- 
charged at  term  leads  to  the  following  additional  observations :  The  color  is  a 
reddish  or  purplish  gray,  varying  in  tint  according  to  the  condition  of  the  blood, 


FIG.  196. — HUMAN  PLACENTA  AT  FULL  TERM,  DOUBLY  INJECTED  TO  SHOW  THE  SUPERFICIAL  DISTRIBU- 
TION OF  THE  BLOOD-VESSELS. 
The  veins  are  drawn  dark  and  lie  deeper  than  the  arteries.     One-half  natural  size. 

and  mottled  between  the  divaricating  blood-vessels  by  patches  and  networks  of 
pale  yellowish  or  flesh  color.  The  light  pattern  is  produced  by  the  tissue  of  the 
villi  shining  through  the  membrane  of  the  chorion.  These  appearances  are  less 
distinct  when  the  placenta,  as  is  usually  the  case,  is  covered  by  the  thin  amnion. 


THE  HUMAN  PLACENTA.  .  339 

The  amnion,  however,  is  very  easily  detached  as  far  as  the  insertion  of  the  um- 
bilical cord,  to  the  end  of  which  it  is  firmly  attached,  but  it  cannot  be  traced 
farther  because  on  the  cord  itself  there  is  no  amnion.  The  blood-vessels  run  out 
in  all  directions  from  the  end  of  the  cord ;  each  vessel  produces  a  ridge  upon  the 
placental  surface,  so  that  its  course  is  readily  followed.  The  arteries  and  veins 
are  more  easily  distinguished  after  double  injection,  as  is  shown  in  figure  196. 

The  two  kinds  of  vessels  do  not  run  together ;  the  arteries  lie  near  the  sur- 
face, just  above  the  veins ;  the  arteries  fork  repeatedly,  until  they  are  represented 
only  by  small  branches  and  fine  vessels;  some  of  the  small  branches  disappear 
quite  suddenly  by  dipping  down  into  the  deeper-lying  tissue  in  order  to  pass  into 
the  villi.  The  veins  (Fig.  196)  are  considerably  larger  than  the  arteries;  they 
branch  in  a  similar  manner,  but  some  of  the  trunks  disappear  from  the  surface 
more  abruptly  than  is  the  case  with  the  arteries.  There  is  the  greatest  possible 
variability  in  the  vessels  of  the  placenta;  I  have  never  seen  two  placentae  with 
vessels  alike.  So  far  as  I  have  observed,  the  insertion  of  the  cord  is  always  ob- 
viously eccentric ;  the  degree  of  eccentricity  is  variable  and  is  easily  seen  to  be 
related  to  the  distribution  of  the  vessels. 

The  insertion  of  the  cord  may  even  be  entirely  outside  the  placenta,  which 
yet  may  otherwise  be  normally  developed.  Such  insertions  are  called  velamen- 
tous.  The  usual  type  is  shown  in  figure  196.  The  arteries  come  down  together 
from  the  cord;  they  usually,  but  not  always,  anastomose  by  a  short  transverse 
vessel,  which  lies  about  half  an  inch  above  the  surface  of  the  placenta;  it  could 
not  be  shown  in  the  figure.  Very  rarely,  if  ever,  are  there  any  arterial  or  venous 
anastomoses  on  the  surface  of  the  placenta.  The  arteries  there  spread  out  in  a 
manner  which  may  be  described  as  roughly  symmetrical.  The  veins  partially 
follow  the  course  of  the  arteries.  When  the  cord  is  inserted  near  the  margin,  the 
symmetry  of  the  placental  vessels  is  greater,  when  the  insertion  is  near  the  cen- 
ter, the  symmetry  is  less,  than  in  the  figure. 

The  reverse  or  uterine  surface  of  the  placenta  is  rough  and  divided  into 
numerous  rounded  oval  or  angular  portions  termed  lobes  or  cotyledons,  as  stated 
above.  These  vary  from  half  an  inch  to  an  inch  and  a  half  in  diameter.  The 
whole  of  this  surface  consists  of  a  thin,  soft,  somewhat  leathery  investment  by  the 
decidual  membrane,  which  dips  down  in  various  parts  to  form  the  grooves  that 
separate  the  cotyledons  from  each  other.  This  layer  is  a  portion  of  the  decidua 
serotina,  which,  as  long  as  the  parts  are  in  situ,  constitutes  the  boundary  between 
the  placenta  and  the  muscular  substance  of  the  uterus,  but  which  at  the  time  of 
labor  becomes  split  asunder,  so  that  while  a  portion  is  carried  off  along  with  the 
placenta  and  constitutes  its  external  membrane,  the  rest  remains  attached  to  the 
inner  surface  of  the  uterus.  If  a  placenta  is  cut  through,  it  is  found  to  consist  of 


340 


HUMAN  UTERUS  AND  FCETAL  APPENDAGES. 


a  spongy  mass  containing  a  large  quantity  of  blood  and  bounded  by  two  mem- 
branes, each  less  than  a  millimeter  thick;  the  upper  one  is  the  chorion,  covered 
by  the  still  thinner  amnion,  and  greatly  thickened  where  the  vessels  lie  in  it ;  the 
lower  one  is  the  decidual  tissue,  together  with  the  ends  of  the  villi  imbedded  in 
it  (cf.  especially  page  335  and  Fig.  194) ;  it  represents  only  a  portion  of  the  deci- 
dua,  the  other  portion  having  remained  upon  the  uterine  wall.  The  spongy  mass 


st 


FIG.  197. — HUMAN  PLACENTA  AFTER  DELIVERY  AT  FULL  TERM. 

A,  Vertical  section  through  the  margin  :  D,  decidua  ;  vi,  aborted  villi  outside  the  placenta ;  Cho,  chorion  ;  Si, 
sinus;  Vi,  placental  villi;  Fib,  fibrin.  B,  Portion  of  A  more  highly  magnified  to  show  the  decidual  tissue 
near  b  :  v,  blood-vessel ;  d,  decidual  cell  with  one  nucleus  ;  d' ' ,  decidual  cell  with  several  nuclei. 

is  found  upon  examination  to  consist  of  an  immense  number  of  tufts  of  fine  rods 
of  tissue,  which  are  irregularly  cylindrical  in  shape.  Further  examination 
shows  that  they  are  twigs  (Fig.  204),  with  rounded  ends  and  springing  from 
branchlets  which  in  their  turn  arise  from  branches,  and  so  on  until  a  large  main 
stem  is  found,  which  starts  from  the  chorion.  This  branching  system  is  richly 
supplied  with  blood  from  the  foetal  vessels  on  the  surface  of  the  placenta.  The 


HISTOLOGY  OF  THE  HUMAN  CH ORION.  341 

villi  are  interwoven  so  that  the  twigs  of  one  branch  are  interlaced  with  those  of 
another,  and  apparently  separate  twigs  may  grow  together  and  their  vessels 
anastomose;  but  on  this  point  we  are  unable  to  speak  positively.  The  villous 
twigs  next  the  surface  of  the  decidua  penetrate  that  tissue  a  slight  distance. 

The  intervillous  spaces  are  filled,  or  nearly  so,  with  blood;  they  form  a  com- 
plex system  of  channels.  The  intervillous  blood  is  maternal.  Farre  says,  in  his 
article  in  Todd's  "Cyclopaedia"  (V.  Suppl.,  page  716),  in  reference  to  the  pla- 
cental  decidua :  ' '  Numerous  valve-like  apertures  are  observed  upon  all  parts  of 
the  surface;  they  are  the  orifices  of  the  veins,  which  have  been  torn  off  from  the 
uterus.  A  probe  passed  into  any  one  of  these,  after  taking  an  oblique  direction, 
enters  at  once  into  the  placental  substance.  Small  arteries  about  half  an  inch  in 
length  are  also  everywhere  observed  imbedded  in  this  layer.  After  making 
several  sharp  spiral  turns  they  likewise  suddenly  open  into  the  placenta";  and 
on  page  719  he  adds :  "These  venous  orifices  occupy  three  situations.  The  first 
and  most  numerous  are  scattered  over  the  inner  side  of  the  general  layer  of  de- 
cidua which  constitutes  the  upper  boundary  of  the  placenta;  the  second  form 
openings  upon  the  sides  of  the  decidual  prolongations  or  dissepiments  which 
separate  the  lobes  (cotyledons)  from  each  other;  while  the  third  lead  directly 
into  the  interrupted  channel  in  the  margin,  termed  the  circular  sinus."  The 
circular  sinus  (Fig.  197,  Si)  is  merely  a  space  at  the  edge  of  the  placenta  which  is 
left  comparatively  free  from  the  villi.  It  is  not  a  continuous  channel,  but  is  inter- 
rupted here  and  there.  Subsequent  writers  have  gone  but  little  beyond  Farre's 
account,  which  has  been  entirely  overlooked  by  most  recent  investigators,  who, 
accordingly,  have  announced  as  new  discoveries  many  facts  known  to  Farre. 
Under  these  circumstances  it  is  interesting  to  direct  renewed  attention  to  Farre's 
masterly  article. 

Histology  of  the  Human  Chorion. 

The  chorion  may  be  preserved  in  Zenker's  or  Tellyesnicky's  fluid  or  in 
Kleinenberg's  picro-sulphuric  acid.  Pieces  may  be  stained  in  toto  with  alum 
cochineal  or  borax  carmine  and  vertical  sections  cut  in  paraffin.  The  sections 
may  be  advantageously  counterstained  with  eosin  or  orange  G. 

For  the  general  history  of  the  chorion  see  page  78.  As  it  is  formed  by  the 
somatopleure,  it  comprises  an  outer  ectoderm  and  an  inner  mesoderm,  which 
latter  comprises  mesenchyma  and  mesothelium. 

The  ectoderm  undergoes  a  very  precocious  growth  producing  a  very  large 
number  of  cells,  which  form  the  thick  trophoblastic  layer  as  described  on  page 
342.  Then  follows  the  stage  in  which,  by  degeneration,  spaces  are  produced  in 
the  trophoblast  into  which  the  blood  of  the  mother  enters  and  circulates;  and  at 


342  HUMAN  UTERUS  AND  FCETAL  APPENDAGES. 

the  same  time  prolongations  of  the  chorionic  mesoderrn  extend  into  the  tropho- 
blast.  The  ectodermal  cells  arrange  themselves  as  a  covering  for  these  mesoder- 
mic  outgrowths  and  so  complete  a  villus.  The  trophoblast  between  the  develop- 
ing villi  entirely  disappears.  The  ectoderm,  which  covers  both  the  villi  and  the 
chorionic  membrane  proper,  consists  of  two  layers,  an  inner  cellular  and  an  outer 
syncytial  layer.  Much  of  the  trophoblast  may  still  remain  for  awhile  around 
and  beyond  the  tips  of  the  villi,  but  it  disappears  rapidly,  probably  during  the 
third  week,  so  that  the  villi  alone  are  left.  The  two-layered  stage  of  the  ecto- 
derm is  only  partially  preserved  during  the  later  development.  Many  parts  of  it 
become  thinned  out  so  as  to  contain  only  one  layer  of  cells,  while  other  parts 
thicken  and  degenerate.  These  changes  may  be  studied  in  sections  of  older 
placentas  (see  Fig.  190). 

The  mesoderm  of  the  chorion  consists  at  first  of  mesenchymal  cells  with  a 
homogeneous  matrix  and  a  layer  of  mesothelium.  In  later  stages  the  mesen- 
chymal  tissue  becomes  partly  fibrillar,  and  it  is  doubtful  whether  the  mesothe- 
lium persists  or  not.  During  the  third  week  we  find  the  chorion  vascular. 
Around  the  larger  blood-vessels  the  jnesoderm  forms  a  more  or  less  distinct 
coat  in  which  the  cells  are  somewhat  more  crowded  together  in  laminae.  After 
the  perivascular  coats  have  acquired  a  certain  thickness  the  cells  of  their  inner 
portions  become  more  elongated,  more  regularly  spindle-shaped,  and  more 
closely  packed  than  those  of  the  outer  layer.  The  transition  from  the  denser  to 
the  looser  tissue  is  gradual.  We  are  perhaps  entitled  to  call  the  denser,  inner 
layer  the  media,  and  the  outer,  looser  layer  the  adventitia,  although  neither  of  the 
layers  has  by  any  means  the  full  histological  differentiation  characteristic  of  the 
like-named  layers  of  the  blood-vessels  of  the  adult.  The  histogenetic  changes  in 
the  chorionic  frondosum  go  further  than  in  the  chorion  laeve,  which  may  be  said 
to  be,  as  it  were,  arrested  in  its  development. 

The  Chorion  with  Trophoblast. 

When  the  chorionic  vesicle  has  an  internal  diameter  of  from  3  to  6  or  7  mm., 
it  will  be  found  to  exhibit  well- developed  trophoblastic  layers.  Such  a  vesicle 
may  be  hardened  in  Zenker's  fluid,  or,  better,  in  Flemming's  or  Hermann's  fluid, 
as  these  produce  at  the  same  time  a  differential  color  (Fig.  198).  The  chorionic 
membrane  is  quite  thin,  and  consists  chiefly  of  mesoderm,  mes,  with  a  covering 
of  ectoderm,  EC,  consisting  of  two  layers  of  cells.  The  mesoderm  extends  down 
to  form  the  core  of  the  villi  shown.  These  villi  are  much  branched  and  are  also 
covered  by  a  layer  of  ectoderm.  At  the  denser  ends  of  the  villi  the  ectoderm  is 
very  much  thickened,  forming  a  great  mass  of  cells,  so  that  the  ectoderm  con- 
nected with  one  villus  is  fused  with  that  of  adjacent  villi,  the  whole  constituting 


THE  CH ORION  WITH  TROPHOBLAST. 


343 


a  large  irregular  mass  of  cells,  Tro.  This  is  the  trophoblast.  In  many  places  it 
has  already  disappeared,  so  that  there  are  spaces,  lac,  in  the  trophoblastic  mass. 
On  the  edges  of  these  spaces  the  trophoblast  is  undergoing  degeneration,  deg,  and 
where  that  is  occurring  it  is  marked  in  the  figure  by  the  deeper  staining  of  the 
degenerated  material.  Upon  examination  with  a  higher  power  (Fig.  199)  it  will 
be  noted  that  the  mesodermic  cells  are  stained  much  more  deeply  than  the 


Mes. 


EC. 


lac. 


Tro. 


FIG.  198. — SECTION  OF  A  VERY  YOUNG  HUMAN  CHORION. 

deg,  Degenerating  ectoderm.    EC,  Epithelial  ectoderm,    lac,  Lacuna  for  maternal  blood.    Mes,  Mesoderm.     Tro, 

Trophoblast.      Vi,  Villi.     X  5°  diams. 

matrix.  They  have  an  elongated  form  and  run  in  various  directions,  more  or 
less  parallel  to  the  epithelium,  EC' .  Many  of  them  are  cut  transversely  or  ob- 
liquely. Aside  from  the  trophoblast,  the  ectoderm  is  everywhere  two-layered. 
The  inner  layer  is  distinctly  cellular,  the  outlines  of  the  cells  being  very 
sharply  marked  and  the  nuclei  being  relatively  large.  In  the  outer  layer,  which 
is  stained  more  darkly,  there  are  no  cell  boundaries  to  be  recognized,  the  struc- 


344 


HUMAN  UTERUS  AND  ECETAL  APPENDAGES. 


ture  being  syncytial.  The  nuclei  are  smaller  and  more  deeply  stained  than  those 
of  the  inner  layer.  In  the  trophoblast  we  find  great  masses  of  cells  somewhat 
similar  to  those  of  the  cellular  layer  upon  the  chorionic  membrane  and  over  the 
surface  of  the  villi,  but  they  are  larger  and  more  lightly  stained.  They  lie 
closely  packed  together;  their  nuclei  are  rounded  in  form,  but  vary  considerably 
in  size  and  shape.  Many  of  them  contain  one  or  two  distinct  spots,  which,  how- 


Ec. 


EC." 


Tro. 


Tro. 


FIG.   199. — PORTION  OF  THE  PRECEDING  FIGURE  MORE  HIGHLY  MAGNIFIED. 

deg,  Degenerating  ectoderm.     Ecf ,  Outer  syncytial  layer  of  ectoderm.     Ecf> ',  Inner  cellular  layer  of  ectoderm. 
mes,  Mesoderm  of  villus.      Tro,  Trophoblast.     X  I5°  diams. 

ever,  are  sometimes  absent.  On  the  edges  of  the  spaces  which  have  been  formed, 
and  sometimes  apparently  in  the  interior  of  the  mass  of  trophoblast,  we  find 
bands  and  lines  of  degenerative  material  in  which  we  can  find  nuclei,  but  no 
distinct  cell  boundaries.  The  substance  between  the  nuclei  is  more  or  less  uni- 
formly granular  in  texture  and  stains  quite  deeply.  The  nuclei  of  the  degenera- 


'\        THE  CHORIONIC  VILLI.  345 

tive  material  vary  extremely  in  appearance.  In  some  cases  they  are  small  and 
stain  rather  deeply,  and  are  then  found  to  be  present  in  more  or  less  considerable 
numbers.  Occasionally,  however,  the  nuclei  are  much  larger,  and  more  rarely 
one  sees  a  nucleus  of  exceptionally  great  diameter. 

Our  knowledge  of  the  human  trophoblast  being  still  very  imperfect,  its  full 
history  is  still  partly  a  matter  of  supposition.  The  appearances  described  indi- 
cate that  the  trophoblast  undergoes  a  rapid  degeneration,  during  which  the  cells 
fuse,  while  their  protoplasm  becomes  a  hyaline  material.  We  must  then  further 
suppose  that  the  degenerated  substance  is  resorbed  and  disappears  altogether. 
Finally,  we  must  assume  that  the  entire  trophoblast  does  not  disappear,  but  that 
enough  is  preserved  to  form  the  permanent  covering  of  the  villi. 

It  may  be  noted  that  the  specimen  on  which  the  above  description  is  based 
agrees  essentially  with  the  specimen  described  by  Siegenbeek  van  Heukelom, 
which  is  regarded  as  normal. 

The  Chorionic  Villi. 

The  villi  may  be  obtained  in  connection  with  the  preparations  of  the  uterus 
and  placenta.  In  order  to  see  the  youngest  stages  of  the  first  villi  it  is  necessary 
to  have  the  chorionic  membrane  of  the  second  or  early  part  of  the  third  week. 
At  this  stage  the  trophoblast  is  present  and  the  first  villi  are  appearing  (compare 
page  342).  To  study  the  growth  and  form  of  the  villi,  single  villi  or  pieces  of 
villi  should  be  snipped  off  from  the  chorion  at  various  stages.  Such  pieces  may 
be  examined  as  opaque  objects  in  alcohol,  or  they  may  be  stained  and  mounted 
as  permanent  preparations.  To  obtain  injected  villi  it  is  best  to  inject  the  pla- 
centa through  one  of  the  arteries  of  the  umbilical  cord,  using  as  the  injecting 
mass,  gelatin  colored  with  carmine  or  Prussian  blue.  Such  injections  are  very 
easily  made. 

Branching  of  the  Villi. — The  formation  of  a  branch  is  usually  initiated  by 
an  outgrowth  of  the  ectoderm.  Branches  grow  very  rapidly;  the  outgrowth 
which  forms  the  branch  occurs  with  every  degree  of  participation  of  the  meso- 
derm.  The  two  extremes  are  first  the  bud,  consisting  wholly  of  epithelium,  which 
may  become  a  process  with  a  long,  thin  pedicle  and  a  thickened  free  end  remain- 
ing sometimes  entirely  without  mesoderm.  Later  the  mesoderm  penetrates  it 
and  completes  the  structure.  Second,  a  thick  bud  with  a  well-developed  cord 
of  connective  tissue  and  having  a  nearly  cylindrical  form.  Between  these 
extremes  every  intermediate  stage  can  be  found.  The  tips  of  the  branches  are 
for  the  most  part  free,  but  some  of  them  come  in  contact  with  the  surfaces  of  the 
decidua  and  penetrate  it  for  a  short  distance.  By  this  means  the  villi  of  the 
embryo  are  attached  to  the  decidua  of  the  mother.  The  villi  do  not  penetrate 


346  HUMAN  UTERUS  AND  FCETAL  APPENDAGES. 

the  glands  of  the  uterus  at  any  period,  as  was  at  one  time  supposed.  The  ecto- 
derm on  the  tip  of  the  villi,  where  it  is  in  contact  with  decidual  tissue,  undergoes 
a  hyaline  degeneration. 

The  shape  of  the  mlli  varies  according  to  the  part  of  the  chorion  and  the  age 
of  the  embryo.  Over  the  chorion  laeve  there  is  first  an  arrest  of  development  and 
a  subsequent  slow  degeneration  of  the  tissues,  which  lose  all  recognizable  organ- 
ization of  their  protoplasm,  and  to  a  large  extent  of  their  nuclei  also.  At  the 
same  time  they  alter  their  shape  (Fig.  200),  becoming  more  and  more  filamen- 
tous. By  the  fourth  month  only  a  few  tapering  threads  with  very  few  branches 
remain.  The  villi  disappear  almost  completely  from  the  chorion  laeve,  except 
near  the  edge  of  the  placenta.  The  mlli  of  the  chorion  frondosum  or  placental 


x9 

FIG.  200. — ABORTING  VILLUS  FROM  THE  HUMAN  FIG.  201. — FRAGMENT  OF  THE  CHORION  OF  FIG. 

CHORION  IVEVE  OF  THE  SECOND  MONTH.  69,  HIGHLY  MAGNIFIED. 

EC,  Ectoderm.     Mes,  Mesoderm.     Vi,  Villus  formed 
wholly  by  ectoderm. 

region,  on  the  contrary,  make  an  enormous  growth.  At  first  they  are  short, 
thick-set  bodies  of  irregular  shape,  as  shown  in  figure  201.  At  twelve  weeks 
their  form  is  extremely  characteristic  (Fig.  202).  The  main  stem  gives  off 
numerous  branches  at  more  or  less  acute  angles,  and  these  again  other  branches, 
until  at  last  the  terminal  twigs  are  reached.  The  branches  are  extremely  irregu- 
lar and  variable,  though  in  general  club-shaped  and  constricted  at  the  base.  The 
branches  may  be  bigger  than  the  trunk  which  bears  them,  or  of  any  less  size.  In 
older  stages  there  is  a  progressive  change.  During  the  fifth  month  we  find  the 
irregularity  of  shape,  though  still  very  marked,  decidedly  less  exaggerated  (Fig. 
203).  The  branches  tend  to  come  off  at  more  nearly  right  angles.  One  finds 
very  numerous  free  ends,  as  of  course  only  a  small  portion  of  the  branches  touch 


THE  CHORION1C  VILLI. 


347 


FIG.  202. — ISOLATED  TERMINAL  BRANCH  OK  A 
VILI.US  FROM  A  HUMAN  CHORION  OF  TWELVE 
WEEKS. 


FIG.  203. — VILLOUS  STEM  FROM  A  HUMAN  PLA- 
CENTA OF  THE  FIFTH  MONTH.     X  9  diams. 


XI9 


FIG.  204. — TERMINAL  BRANCHES  OF  A   VILLUS  FROM  A 

HUMAN  PLACENTA  AT  FULL  TERM. 

The  little  spots  indicate  proliferation  islands  of  the  covering 
epithelium.     Magnified. 


FIG.  205.— PORTION  OF  AN  INJECTED  VIL- 
LUS FROM  A  PLACENTA  OF  ABOUT  FIVE 
MONTHS.  X  2I°  diams. 


348 


HUMAN  UTERUS  AND  FCETAL  APPENDAGES. 


the  decidual  surface.  The  branches,  too,  are  less  out  of  proportion  to  the  stems, 
less  constricted  at  their  bases,  less  awkward  in  form.  The  gradual  changes  con- 
tinue until  at  full  term,  as  shown  by  figure  204,  the  branches  are  long,  slender, 
and  less  closely  set  as  well  as  less  subdivided  than  at  early  stages.  They  have 
nodular  projections  like  branches  arrested  at  the  beginning  of  their  development. 
There  are  numerous  spots  upon  the  surfaces  of  the  villi.  Microscopic  examina- 
tion shows  that  these  spots  are  proliferation  islands,  as  we  may  call  them,  or 

little  thickenings  of  the  ectoderm  with  crowded  nuclei. 
Not  all  the  villi,  however,  have  changed  to  the  slender 
form,  for  some  still  preserve  the  earlier,  clumsier  shapes. 
In  sections  of  placentas  of  different  ages  the  villi  offer 
characteristic  differences;  for  the  younger  the  stage, 
the  fewer  the  total  number  of  branches  and  the  larger 
their  average  size.  The  older  the  placenta,  the  more 
numerous  and  smaller  are  the  branches  as  they  appear 
in  sections  (Fig.  190). 

Injected  Villi. — The  arteries  and  veins  of  the  cho- 
rionic  membrane  enter  the  villi.  After  a  short  course 
in  the  main  stalk  of  a  villus,  the  vessels  give  rise  to 
many  branchlets,  and  gradually  the  character  of  the 
circulation  changes,  until  in  the  smallest  villous  twigs 
there  are  capillaries  only  (Fig.  205).  The  capillaries 
are  remarkable  for  their  large  size,  and  on  this  account 
have  been  interpreted  as  arteries  and  veins  by  some  of 
the  older  writers.  Their  caliber  is  often  sufficient  for 
the  passage  of  from  two  to  six  blood-corpuscles 
abreast.  They  are  very  variable  in  diameter,  and 
also  peculiar  in  exhibiting  sudden  constrictions  and 
dilatations.  In  the  short  knob-like  branches  there  is 
often  only  a  single  capillary  loop,  but  as  the  branch 
becomes  larger  the  number  of  loops  increases  and  they 
form  anastomoses.  In  branches  large  enough  to  serve 

as  a  stem,  some  one  or  two  of  the  vessels  may  be  enlarged.  In  the  branches 
large  enough  to  admit  of  it,  there  are  two  (or  sometimes  only  one)  longi- 
tudinal central  vessels,  the  artery  and  vein  of  a  superficial  network  of  capillaries 
(Fig.  206).  The  formation  of  loops  and  the  large  size  of  the  capillaries  are  not 
especially  characteristic  of  the  villi,  but  of  the  foetal  blood-vessels  in  general. 

The  histology  of  the  villi  is  described  in  the  section  on  the  placenta  in  situ, 
Page  333- 


FIG.  206.  —  PORTION  OF  A 
SMALL  INJECTED  VILLOUS 
STEM  FROM  A  PLACENTA 
OF  ABOUT  FIVE  MONTHS. 
X  105  diams. 


THE  STRUCTURE   OF  THE  AMNION.  349 

The  Structure  of  the  Amnion. 

The  structure  of  the  amnion  may  be  studied  in  sections,  such  as  will  be  ob- 
tained by  the  student  in  connection  with  the  sections  of  the  chicken  and  pig  em- 
bryos. These  preparations  will  show  the  early  stages.  When  the  amnion  is 
first  formed,  it  consists  of  two  layers  of  cells,  both  very  thin,  and  with  somewhat 
widely  separated  nuclei  in  each  layer.  Sometimes  the  nuclei  lie  in  small  groups. 
Between  the  two  layers  is  a  distinct  space.  The  layer  facing  the  embryo  is  a 
continuation  of  the  embryonic  ectoderm,  and  is  more  regular  and  better  defined 
than  the  second  or  mesodermal  layer,  which  is  more  or  less  irregular  and  sends 
at  intervals  protoplasmic  processes  across  the  space  between  the  two  layers 
which  attach  themselves  to  the  ectoderm. 

Human  Amnion  at  Two  Months. — A  section  is  shown  in  figure  207.  The 
ectoderm,  EC,  is  still  very  thin,  but  where  the  nuclei  are  placed  the  layer  is  a  little 
thicker.  The  mesoderm,  on  the  other  hand,  has  become  quite  thick,  and  is 
readily  seen  to  be  separated  into  two  parts,  a  thin  mesothelial  layer,  Msth,  cover- 
ing the  surface  of  the  amnion  toward  the  chorion,  and  a  mesenchymal  layer,  Mes, 


EC 


Mes 


Msth 

FIG.  207.  —  TRANSVERSE  SECTION  OF  A  HUMAN  AMNION  OF  Two  MONTHS. 
Re,  Ectoderm.     Mes,  Mesenchymal  mesoderm.     Msth,  Mesothelium.     X  25°  diams. 

which  makes  up  the  greater  part  of  the  membrane.  Traces  of  fibrillar  structure 
in  the  mesenchyma  are  observable.  No  blood-vessels,  lymphatics,  or  nerves 
have  been  found. 

Human  Amnion  after  the  Fifth  Month.  —  This  should  be  studied  both  in  sec- 
tions and  in  surface  views  of  the  whole  membrane,  small  pieces  being  mounted 
with  the  ectodermal  side  up.  The  preparation  may  be  stained  with  alum 
hematoxylin  and  eosin.  Sections  show  that  the  ectoderm  (Fig.  208,  ect}  has 
grown  somewhat  in  thickness.  Usually  the  cells  are  cuboidal  (Fig.  208,  A), 
each  with  a  rounded  top  in  which  is  situated  the  more  or  less  nearly  spherical 
nucleus.  Sometimes,  however,  the  nuclei  lie  deeper  down.  Less  frequently 
the  epithelium  is  thin  (Fig.  208,  B),  and  its  nuclei,  which  are  transversely  elon- 
gated, lie  further  apart.  As  regards  the  mesoderm,  it  will  be  noticed  that  there 
is  usually,  perhaps  always,  a  layer  of  nearly  homogeneous  basal  substance  or 
matrix  immediately  underneath  the  ectoderm  and  remarkable  for  containing  no 
cells.  Sometimes  the  remaining  portion  of  the  mesoderm  is  broken  up  so  as  to 


350 


HUMAN  UTERUS  AND  FCETAL  APPENDAGES. 


offer  a  fibrillar  structure  (Fig.  208,  A),  and  when  that  is  the  case  we  can  no  longer 
make  out  a  distinct  mesothelial  layer.  At  other  times  the  more  or  less  homo- 
geneous matrix  can  be  seen  through  the  whole  thickness  of  the  amnion  (Fig.  208, 
B),  and  when  this  is  the  case  the  mesothelium,  a,  can  be  readily  identified. 

In  surface  views  the  amniotic  ectoderm  is  seen  to  consist  of  more  or  less 
regularly  distributed  nuclei  with  cell-bodies  connecting  with  one  another  by 
intercellular  bridges  of  protoplasm  (Fig.  209).  The  nuclei,  nu,  are  relatively 
large,  rounded,  and  with  distinct  outlines.  They  have  a  more  or  less  well-marked 
internuclear  network  with  thickened  nodes  and  a  small  number  of  deeply  stained 


FIG.  208. — Two  SECTIONS  OF  THE  HUMAN 
AMNION. 

A,  From  an  embryo  of  eight  months ;  B,  at  term. 
ect,  Ectoderm,  mes,  Mesoderm.  a,  Meso- 
thelium. X  34°  diams. 


FIG.  209. — SURFACE  VIEW  OF  THE  HUMAN  AMNIOTIC 

EPITHELIUM  OF  THE  FOURTH  MONTH. 
//,   Protoplasm,     pr,   Intercellular  processes,     nu,  Nu- 
cleus.    X  I225  diams. 


granules  which  are  probably  chromatin.  Each  nucleus  is  surrounded  by  a  cell- 
body,  pi,  and  the  adjacent  cell-bodies  are  separated  from  one  another  by  clear 
spaces  which  are  crossed  by  threads  of  material,  pr,  stretching  'as  bridges  between 
the  neighboring  cells.  The  protoplasm  is  vacuolated.  The  whole  picture  thus 
leads  to  the  view  that  the  epithelium  is  a  sponge-work  of  protoplasm  somewhat 
condensed  around  each  nucleus.  As  regards  the  mesoderm,  it  is  very  difficult  to 
obtain  clear  pictures  of  the  cells,  though  the  nuclei  can  be  readily  observed. 
They  vary  greatly  in  appearance,  being  sometimes  fairly  regular  and  uniform, 


THE   UMBILICAL   CORD. 


351 


though  always  far  less  so  than  the  nuclei  of  the  mesenchyma  of  the  embryo 
proper.  In  other  cases  (Fig.  210)  the  nuclei  are  exceedingly  irregular;  some 
are  large  with  a  distinct  network,  d;  others  are  smaller  and  differ  but  slightly 
from  the  normal.  Some  are  very  irregular,  b,  others  slightly  irregular,  c,  and 
others  again  strangely  elongated  and  narrow,  a.  Many  other  forms  besides 
those  represented  in  figure  210  may  be  found.  It  has  been  suggested  that  these 


FIG.  210. — NATURAL  GROUP  OF  NUCLEI  FROM  THE  MESODERM  OF  THE  HUMAN  AMNION   OF  THE  FIFTH 
MONTH.     (For  lettering  see  text.)     X  I225  diams. 

varied  shapes  of  the  nuclei  indicate  degenerative  changes,  and,  in  fact,  many  of 
the  nuclei  are  actually  breaking  down,  for  in  some  specimens  every  stage  between 
a  nucleus  and  scattered  granules  can  be  observed,  for  one  may  find  nuclei  with 
distinct  membranes,  without  membranes,  masses  of  granular  matter  stained, 
clusters  of  granules  crowded  together,  and,  finally,  other  clusters  more  or  less 
scattered. 

The  Umbilical  Cord. 

The  umbilical  cord  may  be  best  preserved  in  Zenker's  or  Tellyesnicky's 
fluid.  Transverse  sections  may  be  prepared  in  paraffin  and  stained  with  alum 
hematoxylin  and  eosin,  or  with  Heidenhain's  iron  hematoxylin  and  orange  G; 
or,  if  it  is  desired  to  study  the  development  of  the  connective-tissue  fibrilla,  with 
Mallory's  triple  connective-tissue  stain. 

A  general  description  of  the  umbilical  cord  has  been  given,  pages  109  to 
in,  and  there  two  sections  (Fig.  51)  are  represented  showing  the  structures 
which  appear  in  sections  of  the  umbilical  cord.  At  full  term  some  of  these 
structures  are  still  present  but  somewhat  modified  (Fig.  211),  while  others  have 
been  partly  or  wholly  obliterated.  As  contrasted  with  the  early  stages,  we  find 


352 


HUMAN  UTERUS  AND  FCETAL  APPENDAGES. 


that  the  coelom  is  entirely  obliterated,  that  the  yolk-stalk  has  usually  been  com- 
pletely resorbed,  and  that  only  traces  of  the  allantois  can  be  seen,  Y.  The  blood- 
vessels have  grown;  there  are  two  arteries,  A,  A' ,  and  a  single  vein,  V.  Around 


A' 


FIG.  211. — CROSS-SECTION  OF  A  HUMAN  UMBILICAL 

CORD  AT  TERM. 

A,  Af,  Umbilical  arteries  much  contracted.      V,  Um- 
bilical vein.  Y,  Remnant  of  allantois.  XI2diams. 


FIG.   212. — SECTION  OF  THE  ALLANTOIS  FROM  A 

HUMAN  UMBILICAL  CORD  OF  THREE  MONTHS. 

ent,  Allantoic  entoderm.     mes,  Mesoderm.     X  34° 

dianis. 


FIG.  213. — CONNECTIVE  TISSUE  FROM  THE  UMBILICAL  CORD  OF  A  HUMAN  EMBRYO  OF  21  MM.     STAINED 

WITH  ALUM  COCHINEAL  AND  EOSIN.     X  54°  diams. 

n,  Nucleus,     p,  Protoplasmic  network. 


THE  UMBILICAL   CORD. 


353 


each  of  these  is  a  well-developed  muscular  coat  produced  by  differentiation  of  the 
surrounding  mesenchymal  cells,  which  have  assumed  an  elongated  form  and 
contractile  function.  It  will  be  remembered  that  the  allantois  in  man  is  primi- 
tively a  very  narrow  tubular  diverticulum  which  extends  originally  nearly  to  the 
chorion  (compare  Fig.  71).  As  the  umbilical  cord  lengthens  the  allantois  fails 
to  lengthen  equally.  During  the  second  month  it  increases  very  little  in  diam- 
eter. After  the  second  month  it  appears  in  sections  as  a  small  group  of  epithe- 
lioid  cells  (Fig.  212)  with  distinct  walls,  irregularly  granular  contents,  and  round 


FIG.  214. — CONNECTIVE  TISSUE  FROM  THE  UMBILICAL  CORD  OF  A  HUMAN  EMBRYO  OF  THREE  MONTHS, 

STAINED  WITH  ALUM  COCHINEAL  AND  EOSIN.     X  511  diams. 

f,f,  Cells.    /,  Fibrillse. 

nuclei ;  the  group  may  or  may  not  show  a  remnant  of  the  original  central  cavity. 
Around  the  cells,  ent,  there  is  a  slight  condensation  of  the  connective  tissue,  mes, 
to  form,  as  it  were,  an  envelope. 

The  mesoderm  varies  in  appearance  according  to  the  age  of  the  specimen. 
Its  growth  and  differentiation  are  rapid.     During  the  second  month  it  consists 
merely  of  numerous  cells  (Fig.  213)  imbedded  in  a  clear  substance.     The  cells 
23 


354 


HUMAN  UTERUS  AND  FCETAL  APPENDAGES. 


form  a  complex  network  of  which  the  filaments  and  meshes  are  extremely  varia- 
ble in  size.  The  nuclei  are  oval,  granular,  and  do  not  always  have  accumulations 
of  protoplasm  about  them  forming  main  cell-bodies.  (Compare  description  of 
first  stage  of  the  mesenchyma,  page  65.)  By  the  end  of  .the  third  month  the 
cells  have  assumed  nearly 'their  definite  form  (Fig.  214).  Their  protoplasm  is 
increased  in  amount  and  forms  a  large  body  around  each  nucleus.  The  network 
has  become  simpler  and  coarser,  the  meshes  bigger,  and  the  filaments  fewer  and 
thicker.  In  the  matrix  are  numerous  connective-tissue  fibrillae,  not  yet  disposed 
in  bundles.  In  older  cords  there  is  an  obvious  increase  in  the  number  of  fi- 
brillae and  they  form  wavy  bundles.  In  the  cord  of  yet  older  stages  the  matrix 
also  contains  mucin  which  may  be  stained  by  alum  hematoxylin.  In  such  cords 
so  stained  the  blotch  of  color  appears  in  the  intercellular  spaces. 


FIG.  215. — ECTODERM  OF  AN  UMBILICAL  CORD  OF  A  HUMAN  EMBRYO  OF  THREE  MONTHS. 
EC,  Ectoderm,     tries,  Mesoderm.     c,  Mesenchymal  cell,     a,  Outer  layer  of  ectoderm,     b,  Inner  layer  of  ecto- 
derm.    X  545  diams. 

The  ectoderm  is  at  first  a  single  layer  of  cells,  as  it  is  also  over  the  body  of  the 
embryo,  and  as  it  remains  permanently  over  the  amnion.  At  three  months  we 
find  the  ectoderm  to  be  two-layered,  corresponding  to  the  second  stage  of  the 
epidermis  of  the  embryo.  In  still  older  stages  there  is  slight  increase  in  the 
number  of  layers  of  the  ectoderm,  but  it  never  passes  much  beyond  the  stage  of 
the  embryonic  epidermis  at  the  fourth  month.  Figure  215  is  from  a  cord  at 
three  months.  The  outer  layer,  a,  of  ectodermal  cells  is  granular  and  stains 
much  more  darkly  than  the  inner  layer,  b,  in  which  also  cell  bundles  are  more 
distinct. 

The  Structure  of  the  Human  Yolk=sac. 

The  human  yolk-sac  may  be  preserved  in  Zenker's  or  Tellyesnicky's  fluid, 


THE  HUMAN  YOLK-SAC. 


355 


rnes 


stained  in  toto  with  alum  cochineal,  imbedded  in  paraffin,  and  cut  in  transverse 
sections.  Yolk-sacs  of  the  second  month  are  preferable  for  study. 

The  general  history  of  the  yolk-sac  is  described  on  pages  85  and  89.  It 
becomes  a  pear-shaped  vesicle  which  in  man  attains  its  maximum  diameter 
about  the  end  of  the  fourth  week.  It  then  measures  from  7  to  1 1  mm.  From 
its  pointed  end  runs  the  long  stalk  by  which  it  is  connected  with  the  intestine. 
In  very  early  stages  the  stalk  is  hollow  and  its  cavity  is  lined  by  entoderm. 
But  this  condition  is  soon  obliterated,  the  stalk  becoming  solid  and  the  entoderm 
disappearing.  In  this  condition  we  found  the  yolk-stalk  in  an  embryo  of  2 1  mm. 
(Fig.  51,  A).  The  sac  itself  remains  hollow  (Fig.  216).  It  has  a  lining  of  ento- 
dermal  cells,  En,  and  a  thicker  layer  of  meso- 
derm,  mes,  containing  blood-vessels,  D.  The 
network  of  the  vessels  imparts  a  characteristic 
appearance  to  the  external  or  mesodermic  sur- 
face of  the  yolk-sac.  In  the  earliest  stages  ob- 
served the  entoderm  consisted  of  a  single  layer 
of  cuboidal  cells. 

Transverse  Section  of  a  Yolk-sac  of  about 
Two  Months. — The  contents  of  the  fresh  yolk- 
sac  are  fluid,  but  coagulate  when  the  organ  is 
hardened.  In  the  coagulum  are  found  some 
stained  bodies  which  are  supposed  to  be  yolk 
material.  The  entoderm  has  undergone  prolif- 
eration and  thickening.  These  cells  are  more 
or  less  irregular  and  disposed  in  two  or  three 
layers.  Many  of  the  superficial  cells  are 

stained  deeply  and  have  small  nuclei,  while  the  deeper  lying  cells  are  larger, 
more  lightly  stained,  and  have  larger  nuclei  and  more  distinct  cell  boundaries. 
The  .mesoderm  consists  chiefly  of  somewhat  crowded  mesenchymal  cells,  the 
nuclei  of  which  are  smaller  than  the  entodermal  cells,  and  a  well-marked  layer 
of  mesothelium,  which  forms  the  external  covering  of  the  yolk-sac.  In  the 
mesoderm  appear  relatively  large  blood-vessels,  which  are  usually  found  filled 
with  blood-corpuscles.  The  blood-vessels  have  distinct  endothelial  walls  and  lie 
in  the  part  of  the  mesoderm  toward  the  mesothelium,  so  that  they  are  separated 
somewhat  from  the  entoderm  and  seem  often  to  He  immediately  underneath  the 
mesothelium.  They  are  so  large  that  each  vessel  causes  a  protuberance  upon 
the  yolk-sac. 


FIG.  216. — SECTION  OF  THE   YOLK-SAC 

OF  A  VERY  YOUNG  HUMAN  EMBRYO. 

En,    Entoderm.        mes,    Mesoderm.       v, 

Blood-vessel.—  (After  Fr.  Keibel.} 


CHAPTER    VIII. 
METHODS. 

Measuring  Length  of  Embryos. 

Owing  to  the  many  changes  during  development  in  the  curvature  of  the 
longitudinal  axis  of  the  mammalian  embryo,  it  is  impracticable  to  measure  that 
axis  or  to  employ  any  one  system  of  measurements  to  obtain  comparable  results 
for  all  ages.  For  this  reason  the  best  practice  is  to  measure  in  all  cases  the 
greatest  length  of  the  embryo  in  its  natural  attitude  along  a  straight  line.  The 
limbs  are  not  to  be  included  in  such  measurements.  This  greatest  length  in 
young  stages  will  not  include  the  head  (compare,  for  example,  Fig.  80),  but  in 
most  stages  the  head  would  be  included.  Many  German  authors  employ  the  mea- 
surements introduced  by  His,  which  he  calls  the  Nackenlange,  which  corresponds 
to  the  distance  in  a  straight  line  from  the  neck-bend  to  the  caudal  bend.  As  it  is 
impossible  to  measure  this  distance  in  later  stages,  it  seems  best  not  to  use  it  at 
all.  The  length  of  an  embryo,  as  given  by  German  authors,  is  often  indicated  by 
the  abbreviation  NL.,  and  is,  of  course,  often  different  from  the  measures  used  in 
this  work. 

Methods  of  Reconstruction. 

It  is  often  important  to  obtain  definite  plastic  conceptions  of  the  anatomy 
of  embryos  or  parts  of  embryos  too  small  for  dissection.  To  secure  these  in  the 
best  form,  it  is  necessary  to  reconstruct  either  drawings  or  models  from  sections. 
The  methods  employed  for  these  two  forms  of  reconstruction,  being  different, 
must  be  described  separately. 

Reconstruction  of  Drawings  from  Sections. — To  make  these  reconstructions 
satisfactory,  it  is  indispensable  to  have  an  accurate  outline  of  the  embryo  repre- 
senting it  in  the  point  of  view  to  be  used  for  the  reconstruction  and  enlarged  to 
the  precise  scale  upon  which  the  reconstruction  is  to  be  made.  This  drawing 
must,  of  course,  be  made  before  the  embryo  is  imbedded  and  sectioned.  It  is 
further  necessary  to  know  accurately  the  plane  of  the  sections  and  their  thick- 
ness, and,  finally,  the  total  number  of  sections  in  the  series  must  be  counted.  A 

356 


METHODS  OF  RECONSTRUCTION.  357 

convenient  scale  for  the  reconstruction  of  the  anatomy  of  mammalian  embryos 
is  a  magnification  of  from  16  to  20  diameters. 

Let  us  suppose  that  a  pig  of  1 2  mm.  has  been  drawn  in  a  side  view  magnified 
20  diameters ;  that  the  embryo  has  been  cut  into  900  transverse  sections  and  the 
approximate  plane  of  the  sections  is  known.  It  may  be  more  exactly  deter- 
mined by  the  study  of  the  sections  themselves;  for  instance,  it  may  be  deter- 
mined what  section  is  the  last  to  pass  through  the  surface  of  the  head  in  the 
region  of  the  fore-brain  and  the  last  to  pass  through  the  border  of  the  anterior 
limb.  Then  it  can  be  further  ascertained  through  which  dorsal  segments  these 
two  sections  pass.  By  these  data  the  plane  of  the  two  sections  can  be  accu- 
rately fixed.  Over  the  outline  of  the  embryo  is  now  drawn  a  series  of  lines 
which  represent  the  position  of  the  sections.  It  is  generally  sufficient  to  put  in 
lines  which  represent  only  every  second,  third,  or  even  fourth  section.  If  at  any 
point  where  the  structure  is  complicated  more  details  are  needed,  lines  for  the 
additional  sections  can  be  interpolated.  In  our  supposed  case,  our  lines  repre- 
senting every  fourth  section,  there  would  be  225  parallel  lines,  and  these  should 
be  numbered  to  correspond  to  the  sections  which  they  represent. 

The  outlines  of  the  actual  sections  corresponding  to  the  numbered  lines  in 
the  diagram  must  now  be  made  with  the  camera  lucida.  In  regard  to  these 
great  care  is  necessary,  especially  if,  as  is  likely  to  be  the  case,  the  sections  are 
formed  from  embryos  imbedded  in  paraffin,  because  when  an  embryo  is  so  im- 
bedded it  always  shrinks,  and  after  imbedding  is  smaller  than  before.  The 
shrinkage  seems  to  be  uniform  throughout  and  not  to  disturb  the  topographical 
relations  even  of  the  finest  structures.  Unfortunately  the  shrinkage  is  not  con- 
stant, but  varies  from  specimen  to  specimen,  hence  a  camera  drawing  made  from 
the  sections  and  magnified  20  diameters  will  not  be  of  the  right  size  to  fit  in  the 
diagram,  and  these  drawings  must,  therefore,  be  corrected.  This  may  be  done 
either,  as  is  best,  by  making  the  original  camera  lucida  drawings  of  the  right 
magnification  for  direct  use  in  reconstruction,  or  they  may  be  made  nearly  the 
right  magnification  and  when  they  are  measured  off  the  necessary  correction  may 
be  introduced  by  measuring  them  with  proportional  dividers. 

From  the  camera  lucida  drawings  of  the  single  sections  the  measurements 
are  taken  to  fix  the  position  of  the  parts  in  the  reconstruction. 

For  a  given  section  the  exact  position  in  the  reconstruction  is  given  by 
the  line  on  the  outline  drawing  of  the  embryo  corresponding  to  the  number 
of  the  section.  On  the  drawing  of  the  section  the  distance  of  the  organ 
to  be  recontructed  from  the  point  in  the  section  corresponding  to  the  out- 
line of  the  embryo  is  measured  off,  and  then  marked  upon  the  proper 
line  of  the  reconstruction  diagram.  A  similar  measurement  is  then  taken 


358  METHODS. 

from  the  next  section  and  transferred  to  the  diagram  in  the  same  man- 
ner, and  so  on  with  successive  sections  until  a  series  of  dots  is  obtained  which 
mark  the  outline  of  the  organ.  These  dots  are  then  connected  by  a  continuous 
line,  which  will  indicate  the  form  and  correct  position  of  the  organ.  Simple 
reconstructions  may  be  easily  made  by  these  means.  When,  however,  more 
complicated  reconstructions  are  attempted,  much  judgment  and  skill  are  neces- 
sary in  the  selecting  of  parts  which  may  be  successfully  represented  in  a  single 
drawing,  bearing  in  mind  always  the  point  of  view  which  is  assumed  for  the  re- 
construction, so  that  organs  may  be  correctly  represented  in  their  relative  posi- 
tions, nearer  or  further  from  the  observer  as  he  looks  at  the  drawing.  After  the 
outlines  are  completed  the  shading  of  the  parts  must  be  added,  and  this  often 
requires  a  special  degree  of  skill  and  a  considerable  faculty  of  plastic  imagina- 
tion. As  examples  of  complicated  reconstructions  the  student  is  referred  to 
figures  loo  and  104,  pages  163  and  165. 

Oftentimes  simpler  reconstructions  are  very  helpful  in  which  only  a  few 
sections  are  combined,  as,  for  example,  to  show  the  course  and  branches  of  the 
spinal  nerves  in  young  embryos.  In  such  a  case  the  outline  of  the  middle  section 
of  the  series  proposed  to  be  combined  may  be  selected  to  give  the  outline  of  the 
reconstructed  drawing.  Camera  lucida  drawings  of  this  and  the  neighboring 
sections  to  be  included  should  be  made  of  the  desired  magnification.  The  recon- 
struction itself  may  be  made  upon  tracing  paper,  which  is  laid  successively  over 
the  drawings  of  the  sections  and  the  parts  required  from  each  can  be  added 
upon  the  tracing  paper,  which  will  thus  combine  in  a  single  drawing  the  parts 
intended  to  be  represented.  Reconstructions  of  this  kind  are  easily  made  by 
students  and  are  often  very  instructive. 

Reconstruction  -with  Wax  Plates  by  Born's  Method. — The  basis  of  this  method 
is  to  make  in  wax  a  magnified  reproduction  of  the  single  sections,  representing  in 
the  wax  such  portions  of  the  section  as  it  is  desired  to  reproduce  in  plastic  recon- 
struction. To  this  end  wax  plates  must  be  made  which  represent  a  definite 
magnification  of  the  thickness  of  a  section.  For  working  by  this  method  it  is 
usually  advantageous  to  employ  rather  thick  sections,  say,  of  20  />«.  If  the  mag- 
nification chosen  is  fifty  times,  which  is  practically  often  convenient,  then  the 
wax  plates  should  be  made  fifty  times  20  //  in  thickness,  or  i  mm.  The  most 
convenient  plates  to  work  with  are  those  from  i  to  2  mm.  thick.  Upon  a  wax 
plate  of  the  requisite  thickness  a  camera  lucida  drawing  is  made.  This  may 
be  done  with  a  lithographic  crayon  or  with  a  fine  steel  point.  The  drawings 
must  be  of  exactly  the  right  magnification;  in  the  illustration  chosen,  50  diam- 
eters. Next,  the  wax  plate  is  put  upon  a  glass  or  a  metal  surface  where  it  lies 
perfectly  flat,  and  with  a  sharp  thin-bladed  knife  or  scalpel  the  outline  of  the 


METHODS  OF  RECONSTRUCTION.  359 

organs  which  it  is  intended  to  reconstruct  is  cut  out  as  may  be  desired.  Our  bit 
of  wax  then  represents  a  model  of  the  parts  selected  from  the  section,  and  equally 
magnified  in  the  three  dimensions  of  space.  Wax  plates  made  from  successive 
sections  are  then  piled  up,  one  on  top  of  the  other,  in  the  proper  order.  If 
they  are  rightly  superimposed,  an  operation  which  often  requires  skill  and  judg- 
ment, and  always  requires  the  utmost  care,  then  the  pile  of  plates  will  cor- 
rectly represent  the  form  of  the  parts  included  in  the  reconstruction.  To  fasten 
the  plates  together  it  is  only  necessary  to  pass  a  warm  metal  instrument  over  the 
edges  of  the  plates,  enough  to  melt  the  wax  a  little.  With  proper  care  this  may 
readily  be  accomplished  without  destroying  the  surface  modelling  of  the  recon- 
struction. 

The  simplest  method  of  making  wax  plates  is  to  have  a  large  tin  pan  with 
vertical  sides.  This  is  filled  with  hot  water  and  melted  beeswax  is  poured  on  the 
surface  of  the  water  and  allowed  to  cool.  Plates  of  sufficiently  exact  and  even 
thickness  may  be  cast  in  this  way,  provided  the  operation  is  carried  out  in  a  quiet 
place  so  that  the  surface  of  the  water  is  not  disturbed  while  the  wax  is  hardening. 
It  will  be  found  convenient  to  have  a  large  plate  of  iron,  not  less  than  \  of  an 
inch  in  thickness,  which  may  be  placed  upon  supporters.  The  tin  pan  should  be 
set  upon  this  plate  and  the  plate  heated  by  lamps  below  in  order  to  keep  the 
water  hot  enough  to  allow  the  wax  to  spread  evenly  over  the  surface  of  the  water. 
The  water  must  be  freed  from  air  before  the  wax  is  poured  in,  but  must  not  be 
allowed  to  boil  after  the  wax  has  been  added.  If  bubbles  appear  in  the  wax 
plate,  they  may  be  removed  while  the  wax  is  still  hot  by  directing  the  blue  flame 
from  a  Bunsen  burner  down  upon  them.  If  the  pan  is  heated  directly  without 
the  iron  plate,  it  is  sure  to  warp  and  become  unfit  for  use.  Thin  iron  plates  are 
also  liable  to  be  warped. 

To  determine  the  thickness  of  the  plates  cast  as  described  we  proceed  em- 
pirically. A  weighed  quantity  of  wax  is  melted  and  poured  into  the  pan.  After 
the  plate  has  solidified  it  is  removed  by  cutting  it  free  from  the  edges  of  the  pan, 
and  the  thickness  of  the  plate  is  then  measured  at  various  points  by  micrometer 
callipers.  From  these  data  it  is  easy  to  calculate  exactly  what  thickness  of  plate 
one  gram  of  beeswax  represents.  To  get  accurate  results  it  is  advisable  to  cast 
several  plates  of  varying  thickness  and  determine  the  average  for  one  gram  in 
that  way.  Having  determined  what  one  gram  represents  in  thickness,  it  be- 
comes thereafter  only  necessary  to  weigh  out  the  proper  number  of  grams  in 
order  to  obtain  any  desired  thickness  of  wax  plate.  It  will  be  found  advantage- 
ous to  filter  the  wax  before  using  it.  This  may  easily  be  done  by  a  double  hot- 
water  filter.  Such  a  filter  may  be  made  of  copper.  It  is  desirable  to  connect 
it  with  a  Mariotti's  flask  to  maintain  a  constant  water  level. 


360  METHODS. 

Directions  for  Orienting  Serial  Sections  of  Embryos.  (NOTE:  The  lower 
edge  of  the  ribbon  is  the  one  to  the  left,  when  the  observer  has  the  object 
between  himself  and  the  knife.) 

1 .  Transverse  Series. 

Normal  thickness:  10  p. 

Dorsal  surface  to  be  toward  the  lower  edge  of  the  ribbon. 
Series  to  begin  with  the  head. 
In  cutting,  the  left  side  of  the  embryo  must  strike  the  knife  first. 

2.  Sagittal  Series. 

Normal  thickness:  Small  embryos,   10  //. 

Medium       "         15  IJL. 

Large  20  //. 

The  head  of  the  embryo  to  be  toward  the  lower  edge  of  the  ribbon. 
Series  to  begin  with  the  right  side. 
In  cutting,  the  ventral  side  of  the  embryo  must  strike  the  knife  first. 

3.  Frontal  Sections. 

Normal  thickness:  Small  embryos,  10  //. 
Medium  "  15  //. 
Large  "  20  p. 

The  head  of  the  embryo  is  to  be  toward  the  lower  edge  of  the  ribbon. 
The  series  is  to  begin  with  the  ventral  side. 

In  cutting,  the  left  side  of  the  embryo  must  strike  the  knife  first. 
In  mounting  leave  space  for  the  label  at  the  left-hand  end  of  the  slide. 
Keep  the  sections  in  the  order  cut.     Arrange  them  on  the  slides  in  the  sequence 
of  ordinary  written  lines. 

Microtomes. 

There  are  many  forms  of  microtome  which  may  be  used  with  good  results 
and  which  will  work  very  satisfactorily  for  making  sections  of  small  objects.  The 
cutting  of  larger  objects,  such  as  pig  embryos  of  from  15  to  20  mm.,  and  of  pieces 
of  the  uterus  or  other  organs,  is  more  difficult,  and  microtomes  which  work  satis- 
factorily with  small  objects  often  fail  to  give  good  even  sections  of  more  difficult 
objects.  For  embryological  work  a  microtome  ought,  therefore,  to  be  selected 
which  will  give  perfectly  regular  sections  in  long  series  of  any  desired  thickness 
from  i  up  to  25  mikrons.  It  is  also  desirable  for  economy  of  time  to  have  a  mi- 
crotome which  works  automatically.  ,  These  considerations  lead  the  author  to 
recommend  for  embryological  use  especially  two  forms  of  microtome  made  by 
Messrs.  Bausch  &  Lomb,  of  Rochester,  N.  Y.,  and  designated  by  them  as  the 
"precision"  and  "rotary"  microtomes. 


MICROTOMES. 


361 


The  precision  microtome  (Fig.  217)  consists,  first,  of  an  upper  square  form 
upon  which  the  knife  may  be  clamped  in  any  desired  position;  second,  of  two 
horizontal  ways  upon  which  moves  the  carriage  which  bears  the  object-holder; 
and,  third,  of  a  micrometer  screw  with  an  automatic  feeding  contrivance  on  the 
under  side  of  the  movable  carriage.  The  construction  is  very  solid  and  great 
rigidity  of  the  parts  is  secured.  The  microtome  may  be  used  for  either  paraffin 
or  celloidin  cutting.  According  to  the  author's  experience,  this  microtome 


FIG.  217. — THE  PRECISION  MICROTOME. 

considerably  surpasses  all  other  types  in  the  accuracy  of  the  work  which  may  be 
done  with  it.  The  rotary  microtome  was  originally  made  in  Germany,  and 
various  patterns  have  been  put  upon  the  market  by  German,  French,  English, 
and  American  manufacturers.  The  new  pattern  recently  introduced  by  Messrs. 
Bausch  &  Lomb  embodies  a  considerable  number  of  improvements,  which  render 
the  instrument  (Fig.  218)  very  desirable  for  general  laboratory  use.  It  works 
with  accuracy,  is  very  easy  to  manipulate,  and  cuts  sections  with  extreme 


362 


METHODS. 


rapidity.  It  is  adapted  only  for  paraffin  work.  For  the  general  use  of  students, 
in  elementary  courses  especially,  this  microtome  is  to  be  preferred  to  the  "  preci- 
sion," as  it  requires  less  care  and  works  more  rapidly.  A  single  rotary  micro- 
tome will  be  found  sufficient  for  a  class  of  from  twenty  to  thirty  students  in 
embryology. 

The  microtome  is  an  instrument  of  precision,  which  implies  that  it  must  be 
treated  with  extreme  delicacy  and  kept  most  scrupulously  clean.  It  will  be 
found  usually,  when  complaint  is  made  against  the  microtome,  that  the  com- 
plaint is  misdirected,  and  ought  to  be  not  against  the  machine,  but  against  the 


FIG.  218. — THE  AUTOMATIC  ROTARY  MICROTOME. 


owner.  The  modern  microtome  necessarily  has  several  adjustments,  every  one 
of  which  must  be  exact  and  secure.  If  any  one  of  them  is  imperfect  and  insecure, 
if  any  of  the  movable  parts  are  allowed  to  become  corroded,  or  gummed  up  with 
oil,  or  loose,  or  clogged  with  dust  or  dirt  of  any  kind,  the  microtome  will  not  and 
cannot  work  as  an  instrument  of  precision.  The  knife  used  for  cutting  ought  to 
be  regarded  as  an  integral  part  of  the  microtome  and  as  its  most  delicate  and 
easily  injured  part.  A  perfect  knife-edge  is  the  greatest  treasure  of  the  micro- 
tomist.  To  sharpen  the  knife  satisfactorily  for  fine  section  cutting  is  a  really 
serious  difficulty.  A  skilful  person,  however,  may  get  a  good  edge  by  using  the 


METHODS  OF  HARDENING  AND  PRESERVING.  363 

very  finest  grade  of  oil-stone.  No  oil  should  be  used,  but  instead  a  mixture  of 
equal  parts  of  glycerin  and  water.  Before  the  knife  is  honed  it  must  be  made  as 
clean  as  possible.  The  oil-stone  itself  also  must  be  cleaned  with  equal  care,  and 
the  mixture  of  glycerin  and  water  should,  if  necessary,  be  filtered  before  using 
to  keep  it  free  from  dirt.  A  single  particle  of  dirt  may  be  the  cause  of  making 
many  microscopic  notches  in  the  edge  of  a  knife.  A  knife  is  well  sharpened 
when  its  edge  appears  smooth  and  straight  under  a  magnifying  power  of  twenty- 
five  diameters. 

The  microtome  knife  should  be  as  unlike  a  razor  as  possible.  It  must  have 
a  very  thick  back  and  be  as  heavy  and  rigid  as  practicable,  so  that  the  actual 
cutting-edge  may  be  as  steady  and  inflexible  as  it  can  be  made.  Knives  of  suit- 
ably heavy  construction  are  now  .furnished  with  all  the  best  microtomes. 

Methods  of  Hardening  and  Preserving. 

The  two  most  generally  useful  methods  for  preserving  embryos  are  with 
Zenker's  and  Telly esnicky's  fluids.  Good  results  may  be  had  with  the  other 
reagents.  Specimens  preserved  with  picro-sulphuric  acid  have  the  advantage  of 
staining  readily.  To  study  the  medullary  sheaths  of  nerve-fibers,  as  is  necessary 
to  follow  the  development  of  the  fiber  tracts  in  later  stages,  the  specimens  must 
be  preserved  in  Miiller's  fluid.  Flemming's  and  Hermann's  fluid  are  valuable, 
especially  for  cytological  study,  but  are  applicable  only  to  small  pieces. 

i.  ZENKER'S  FLUID. 

'Formula  :     Corrosive  sublimate, 5  gm- 

Potassium  bichromate, I  gm. 

Sodium  sulphate, I  gm. 

Water, *.    .' loo  c.c. 

Add  5  c.c.  of  glacial  acetic  acid  to  the  fluid  immediately  before  using. 

The  fluid  does  not  have  great  penetrating  power,  but  may  be  used  for  embryos 
of  25  mm.  The  amount  of  fluid  should  be  from  ten  to  twenty  times  the  volume 
of  the  specimen,  and  better  results  are  obtained  if  the  fluid  is  changed  after  a  few 
hours.  Chicks  of  the  first  and  second  days  are  hardened  in  two  to  four  hours; 
embryos  of  6  to  8  mm.  in  eight  to  ten  hours;  embryos  of  12  mm.  in  twenty-four 
hours;  larger  embryos  in  thirty  to  forty  hours.  After  the  proper  interval  in 
Zenker's  fluid  the  specimens  must  be  removed  and  washed  in  running  water  for 
twelve  to  twenty-four  hours.  Transfer  to  50  per  cent,  alcohol  for  one  to  three 
hours,  then  to  60  per  cent.,  70  per  cent.,  and  80  per  cent.  It  is  indispensable  to 
remove  now  the  excess  of  corrosive  sublimate  by  adding  sufficient  tincture  of 
iodine  to  give  the  alcohol  the  color  of  port  wine ;  if  the  iodine  disappears,  it  must 
be  renewed.  After  from  one  to  three  days,  according  to  the  size  of  the  specimen , 


364  METHODS. 

transfer  it  to  fresh  80  per  cent,  alcohol,  which  must  be  changed  until  it  no  longer 
extracts  any  iodine  from  the  specimen. 

2.  TELLYESNICKY'S  FLUID. 

Formula:     Bichromate  of  potassium, 3  gm. 

Water, 100  c.c. 

Immediately  before  using  add  5  c.c.  glacial  acetic  acid. 

This  reagent  is  employed  in  the  same  manner  as  Zenker's  fluid,  except  that  the 
treatment  with  iodine  is  omitted. 

3.  PlCRO-SULPHURIC  ACID. 

Formula :     Picric  acid,      ...        ' , I  gm. 

Sulphuric  acid, 6  c.c. 

Water, 1000  c.c. 

Specimens  are  kept  in  the  fluid  from  four  to  twenty-four  hours,  not  longer,  ac- 
cording to  their  size;  transfer  to  30  per  cent,  alcohol  for  one  hour,  to  50  per  cent, 
alcohol  for  one  to  two  hours,  to  60  per  cent,  alcohol  for  twelve  hours,  and  finally 
to  70  per  cent,  alcohol,  which  must  be  changed  daily  until  it  no  longer  shows  even 
a  trace  of  yellow  discoloration  by  picric  acid. 

4.  MULLER'S  FLUID. 

Formula:     Bichromate  of  potassium, 20  gm. 

Sulphate  of  sodium, 10  gm. 

Water, 1000  c.c. 

Miiller's  fluid  is  a  valuable  reagent,  and  for  the  study  of  the  later  stages  of  the 
nervous  system  indispensable.  The  objections  to  its  use  are  that  it  requires  a 
long  time  to  act,  that  it  renders  the  specimens  brittle,  and  makes  them  some- 
what difficult  to  stain.  It  must  be  used  in  large  quantities  and  be  frequently 
changed,  and  allowed  to  act  on  the  specimens  from  three  to  eight  weeks  according 
to  their  size.  The  appearance  of  a  film  or  scum  indicates  that  the  fluid  needs  to 
be  changed. 

5.  PARKER'S  FLUID. 

Formula:*     70  per  cent,  alcohol, loo  c.c. 

Formaldehyde, I  c.c. 

Very  convenient  when  a  simple  and  expeditious  preservative  is  necessary.  The 
specimens  are  placed  in  the  fluid,  which  ought  to  be  renewed  in  a  few  hours. 
They  may  be  kept  permanently  in  the  fluid,  but  it  is  desirable,  before  using  them 

*  Differs  slightly  from  the  original  formula. 


DIRECTIONS  FOR  IMBEDDING.  365 

for  study,  to  remove  the  formaldehyde  by  treating  them  with  fresh  70  per  cent, 
alcohol. 

6.  FLEMMING'S  FLUID. 

Formula;     I  per  cent,  solution  of  chromic  acid, 50  c.c. 

2  per  cent,  solution  of  osmic  acid, 12  c.c. 

Glacial  acetic  acid, 3  c.c. 

The  solution  must  be  used  freshly  made,  and  must  not  be  kept  in  the  dark.  The 
specimens  must  be  of  small  size  and  as  fresh  as  possible.  They  are  kept  in  the 
fluid  from  twenty-four  to  forty-eight  hours,  then  washed  in  running  water  from 
four  to  twenty-four  hours,  then  transferred  to  alcohols  of  gradually  increasing 
strength.  The  fluid  is  useful  chiefly  for  cytological  work. 

7.  HERMANN'S  FLUID. 

Formula  :     I  per  cent,  platinum  chloride  in  distilled  water, 60  c.c. 

2  per  cent,  osmic  acid  in  distilled  water, 8  c.c. 

Glacial  acetic  acid, 4  c.c. 

Used  in  the  same  manner  and  with  the  same  precautions  as  No.  6. 

Preservation  in  Alcohol. 

When  a  specimen  is  first  put  into  alcohol,  it  should  be  transferred  gradually, 
being  put  first  in  30  or  50  per  cent,  alcohol  for  an  hour  or  more,  then  into  60  per 
cent,  for  several  hours,  70  per  cent,  for  twelve  to  twenty-four  hours,  and  finally 
into  80  per  cent.,  in  which  it  should  be  kept  until  required  for  use.  If  the  speci- 
men is  to  be  sectioned,  it  must  be  placed  in  95  per  cent,  alcohol,  which  must  be 
renewed  at  least  once,  and  be  allowed  to  act  for  twenty-four  hours  or  more,  unless 
the  specimen  is  very  small,  when  a  somewhat  shorter  time  may  suffice. 

Directions  for  Imbedding  Specimens  to  be  Microtomed. 

A.   To  Imbed  in  Paraffin: 

-I.  Stain  in  toto.     (Seepages  367  and  368.) 

2.  Dehydrate  in  alcohol  from  three  to  twenty-four  hours. 

3.  Place  in  oil  of  cloves  and  turpentine  (equal  parts),  one  to  twenty-four 
hours. 

4.  Place  in  fresh  cloves  and  turpentine  for  one  to  twenty-four  hours. 

5.  Place  in  soft  paraffin  at  54°  C.  for  thirty  to  ninety  minutes. 

6.  Place  in  hard  paraffin  at  54°  C.  for  thirty  to  ninety  minutes. 

7.  Warm  a  glass  plate  to  about  70°  C. ;  place  on  it  a  paper  tray  or  metal 
imbedding  frame ;  fill  the  box  with  hard  paraffin  at  54°  C.     Warm  a  spatula  and 
with  it  remove  the  specimen  to  the  tray  or  frame,  and  arrange  it  in  a  proper  posi- 


366  METHODS. 

tion.     As  soon  as  the  paraffin  has  set,  chill  it  rapidly  with  cold  water,  otherwise 
the  paraffin  is  likely  to  crystallize  and  therefore  to  cut  badly. 
B.   To  Imbed  in  Celloidin: 

1.  Dehydrate  the  mass  thoroughly  in  95  per  cent,  alcohol,  four  to  twenty- 
four  hours. 

2.  Place  mass  for  twenty-four  hours  in  alcohol  and  ether,  equal  parts. 

3.  Place  mass  in  thin  syrupy  solution  of  celloidin  in  equal  parts  of  ether  and 
alcohol  for  twenty-four  hours. 

4.  Place  mass  in  thick  viscid  solution  of  celloidin  in  equal  parts  of  ether  and 
alcohol  for  twenty-four  hours. 

5.  Set  mass  on  block  of  vulcanite,  compressed  fiber,  or  maple- wood,  and  as 
soon  as  a  film  has  formed  over  the  surface  of  the  celloidin  (two  to  five  minutes)— 

6.  Immerse  in  80  per  cent,  alcohol  for  twenty-four  hours. 

7.  Cut. 

Method  of  Mounting  Paraffin  Sections. 

The  student  is  advised  to  use  slides  40  x  76  mm.  and  from  1.75  to  2  mm. 
thick.  The  thick  slides  are  much  better  than  the  thin  ones  recommended  by 
dealers.  The  cover-glasses  ought  to  be  35  x  50  mm.  and  o.  1 7  to  o.  18  mm. ;  they 
may  be  readily  obtained  from  German  makers,  but  are  difficult  to  secure  from 
American  manufacturers  except  at  an  exorbitant  price. 

The  serial  order  of  the  sections  should  be  preserved  with  the  utmost  care, 
and  time  spent  in  arranging  the  sections  in  straight  rows  will  be  found  to  be  time 
saved. 

The  albumen-glycerin  methods  of  fastening  the  sections  to  the  slide  will  be 
found  satisfactory. 

Formula .:     Take  the   white   of    one  fresh    egg,   beat  slightly  until  equally  fluid,  filter  it    (it  will  take  about 
twenty-four  hours)  and  add  an  equal  amount  of  glycerin.  To  this  fluid  add  a  small  piece  of  camphor. 

1.  Clean  the  slide  thoroughly. 

2.  Put  on  a  small  drop  of  albumen  solution. 

3.  Spread  it  out  very  thin  with  the  finger. 

4.  Add  five  or  six  drops  of  distilled  water,  which  must  flow  evenly  over  the 
coating  of  albumen. 

5.  Place  the  sections  on  slide  in  regular  rows. 

6.  Warm  the  slides  gently  over  an  alcohol  flame,  to  allow  the  sections  to 
flatten.     The  paraffin  must  not  melt. 

7.  Place  the  slides  in  oven  for  twelve  to  twenty-four  hours  to  evaporate 
the  water  completely. 


METHODS  OF  STAINING.  367 

8.  Dissolve  off  the  paraffin  in  turpentine. 

9.  Put  in  absolute  alcohol  for  three  to  five  minutes. 

10.  Clear  in  turpentine  or  chloroform. 

11.  Mount  in  dammar  varnish.     (Canada  balsam  is  undesirable,  because  it 
becomes  much  discolored  with  age.) 

Methods  of  Staining. 

In  embryological  work  the  specimens  are  usually  stained  in  toto  before  im- 
bedding, either  with  alum  cochineal  or  with  borax  carmine,  the  former  being  the 
more  generally  useful  stain.  Staining  on  the  slide  is  also  much  used  either  to 
secure  a  counterstain  after  the  in  toto  coloration  or  to  secure  some  special  result. 
For  counterstains  eosin,  Lyons  blue,  and  orange  G  are  particularly  recom- 
mended. A  few  of  the  most  important  special  stains  are  given  below. 

1.  ALUM  COCHINEAL. 

Formula  :     Powdered  cochineal, , 6  gm. 

Potassic  alum, 6  gm. 

Water, 80  c.c. 

Boil  vigorously  for  twenty  minutes;  allow  the  fluid  to  settle;  decant  the  clear 
fluid;  add  more  water  and  boil  again;  filter  the  entire  solution  and  evaporate 
the  filtrate  to  80  c.c. ;  add  a  small  piece  of  thymol  or  camphor  to  prevent  the 
growth  of  fungi.  Alum  cochineal  is,  on  the  whole,  the  best  reagent  fo'r  in  toto 
staining,  as  it  will  penetrate  quite  large  objects  and  color  them  uniformly 
throughout,  and  gives  a  good  differentiation  of  the  tissues. 

For  in  toto  staining  place  the  specimen  in  water  until  it  sinks ;  then  transfer 
it  to  the  cochineal  for  twenty-four  hours,  or  for  large  specimens  longer;  the 
depth  of  the  stain  will  depend  upon  the  strength  of  the  solution;  transfer  to 
clean  water  for  fifteen  to  twenty  minutes  to  extract  the  alum,  which  otherwise 
will  crystallize  in  the  tissues  when  the  specimen  is  placed  in  alcohol ;  the  object 
must  not  be  left  too  long  in  water,  because  it  extracts  the  color  also ;  put  in  50 
per  cent,  alcohol  for  one  hour,  then  successively  in  70  per  cent.,  80  per  cent.,  and 
95  per  cent.,  when  the  specimen  will  be  ready  for  imbedding. 

2.  BORAX  CARMINE. 

Formula  :     Best  carmine, 3  gm> 

Borax 2  gm. 

Water, 50  c.c. 

Boil  for  twenty  minutes ;  allow  the  solution  to  cool ;  add  water  enough  to  restore 
that  lost  by  evaporation,  then  add  50  c.c.  of  70  per  cent,  alcohol,  let  the  solution 


368  METHODS. 

stand  twenty-four  hours ;  filter.     Borax  carmine  gives  a  good  nuclear  stain  and 
may  be  advantageously  supplemented  by  counterstains. 

For  in  toto  staining  place  the  specimen  in  water  until  it  sinks ;  transfer  to  the 
carmine  for  twenty-four  hours,  or  longer  for  large  specimens ;  wash  in  water  for 
five  minutes;  then  place  it  in  70  per  cent,  alcohol,  to  every  100  c.c.  of  which  2  c.c. 
of  hydrochloric  acid  have  been  added;  after  one  hour  transfer  to  fresh  70  per 
cent,  alcohol,  which  must  be  renewed  in  an  hour  or  two,  and  finally  transfer  to 
80  per  cent,  and  95  per  cent,  alcohol,  and  the  specimen  will  be  ready  to  imbed. 

3.  COUNTERSTAINS  are  used  either  with  celloidin  sections  treated  singly,  or 
with  paraffin  sections  after  they  have  been  fastened  on  the  slide.     The  three  here 
recommended  are  alcoholic  solutions,  and  the  method  of  using  is  the  same  for  all. 

For  staining  paraffin  sections  on  the  slide  it  is  convenient  to  have  eight  jars 
or  dishes  large  enough  to  hold  a  slide.  The  slide  is  transferred  from  jar  to  jar  in 
the  order  below,  being  allowed  to  remain  in  each  jar  a  few  minutes.  The  very 
most  scrupulous  care  is  necessary  to  keep  all  the  fluids  clean,  and  it  is  indispensable 
to  filter  them  frequently;  the  sections  on  the  slide"  catch  and  hold  the  particles 
floating  in  the  reagents  when  they  are  not  clean. 

Order  of  jars:  i.  Xylol. 

2.  Xylol. 

3.  Xylol  and  absolute  alcohol,  equal  parts. 

4.  Absolute  alcohol. 

5.  Counterstain. 

6.  Alcohol  of  95  per  cent. 

7.  Absolute  alcohol. 

8.  Xylol. 

Eosin  Formula:  2  per  cent,  in  95  per  cent,  alcohol. 

Orange  G  formula:     l  per  cent,  in  95  per  cent,  alcohol. 
Lyons  blue  Formula;    I  per  cent,  in  95  per  cent,  alcohol. 

4.  HEIDENHAIN'S  IRON  HEMATOXYUN. 

Formula  I :  Iron  alum, 2  gm. 

Distilled  water, looc.c. 

II :   Hematoxylin  crystals, I  gm. 

95  per  cent,  alcohol, 10  c.c. 

Distilled  water, 90  c.c. 

1.  Place  sections  in  the  iron  solution  for  thirty  to  sixty  minutes.     (Speci- 
mens hardened  with  Flemming's  or  Hermann's  fluid  require  longer  than  speci- 
mens from  Zenker's  or  Tellyesnicky's  fluid.) 

2.  Wash  quickly  in  water. 


METHODS  OF  STAINING.  369 

3.  Transfer  to  the  hematoxylin  solution  for  five  to  ten  minutes. 

4.  Wash  in  tap-water. 

5.  Decolorize  in  the  iron  solution. 

6.  Wash  thoroughly  in  tap- water. 

7.  Dehydrate  and  mount. 

This  stain  is  useful  for  cytological  work,  the  study  of  cell  division,  etc.  The 
preparations  are  often  improved  by  counterstaining  with  orange  G. 

5.  BEALE'S  CARMINE. 

Formula  :     Best  carmine, I  gm. 

Ammonia, 3  c.c. 

Pure  Glycerin, 96  c.c. 

Distilled  water, 96  c.c. 

Alcohol,  95  per  cent., 24  c.c. 

Dissolve  in  ammonia  plus  part  of  the  water,  add  the  rest  of  the  water,  and 
allow  the  solution  to  stand  in  an  open  dish  until  the  ammonia  is  nearly  all  driven 
off.  Then  add  the  alcohol  and  glycerin.  For  use  dilute  with  an  equal  part  of 
glycerin.  Stain  for  twenty-four  hours  in  an  open  dish,  which,  together  with  a 
second  open  dish  containing  acetic  acid,  is  placed  under  a  bell-jar;  wash  the 
sections  thoroughly  in  water  and  then  in  very  weak  hydrochloric  acid  (i  c.c.  to 
500  c.c.  water),  and  again  in  water. 

Beale's  carmine  is  especially  valuable  for  the  study  of  the  central  nervous 
system  and  of  the  placenta. 

6.  WEIGERT'S  COPPER  HEMATOXYUN. 

formula  I :  Copper  solution  : 

Acetate  of  copper  in  saturated  aqueous  solution. 
II :   Hematoxylin  solution  : 

Hematoxylin  crystals, 2  gm. 

95  per  cent,  alcohol, 20  c.c. 

Distilled  water, 80  c.c. 

This  stain  is  indispensable  for  the  study  of  the  nervous  system  after  the 
medullary  sheaths  have  begun  to  develop ;  the  specimens  must  be  preserved  in 
Miiller's  fluid.  The  method  is  also  valuable  for  the  study  of  the  placenta  and 
uterus. 

1 .  Place  the  sections  in  water. 

2.  Place  the  sections  in  the  copper  solution  for  twenty-four  hours. 

3.  Wash  quickly  in  water. 

4.  Put  them  in  the  hematoxylin  solution  for  five  to  ten  minutes.     The  sec- 
tions should  turn  a  deep  blue-black. 

24 


370  METHODS. 

5.  Wash  thoroughly  in  water. 

6.  Decolorize  in  the  iron  solution ;  the  section  must  be  gently  moved  about 
to  secure  an  even  decolorization.     When  part  of  the  section  shows  a  brown  color, 
it^ should  be  removed  and  examined. 

7.  Wash  thoroughly  in  water  to  remove  the  iron  solution,  no  trace  of  which 
can  be  left  without  ruining  the  specimen. 

8.  Dehydrate  with  alcohol  and  mount  at  once  in  dammar. 

7.  MAU,ORY'S  TRIPLE  CONNECTIVE-TISSUE  STAIN. 

formula  I :  Acid  fuchsine, i.o  gm. 

Distilled  water, looo.o  c.c. 

II :  Phosphomolybdic  acid, I.o  gm. 

Distilled  water, 100.0  c.c. 

Ill:  Aniline  blue,  soluble  in  water, 0.5  gm. 

Orange  G 2.0  gm. 

Oxalic  acid, 2.0  gm. 

Distilled  water, . 100.0  c.c. 

1.  Preserve  in  corrosive  sublimate  or  Zenker's  fluid. 

2.  Stain  the  sections  in  the  fuchsine  solution  one  to  three  minutes. 

3.  Wash  in  water. 

4.  Place  in  the  phosphomolybdic  solution  one  minute. 

5.  Wash  in  two  changes  of  water. 

6.  Stain  in  blue  and  orange  solution  two  to  twenty  minutes. 

7.  Wash  in  water. 

8.  Dehydrate  in  95  per  cent,  alcohol  and  mount  in  dammar. 

This  method  gives  a  perfect  differential  stain  of  connective-tissue  fibrils, 
and  it  is  to  be  used  whenever  the  fibrils  are  to  be  especially  studied. 


NDEX. 


A. 


Acoustic  nerve,  pig,  12  mm.,  213 
Allantois,  anlage  of,  chick,  289,  290 

early  human,  136 

general  account,  103 

in  umbilical  cord,  no,  351 

mesothelial  villi  of,  222,  253 

pig,  9  mm.,  221 

pig,  17  mm.,  239 

pig,  24  mm.,  268 

relation  to  chorion  in  ungulates,  105 
Alum  cochineal,  367 
Amiurus,  section  of,  108 
Amnio-cardiac  vesicles,  chick,  295,  300 
Amnion,  anlage  of,  78 

at  seven  months,  327 

chick,  278,  282 

fold  of,  286,  287 
raphe  of,  278,  282,  284 

early  human,  134 

origin  of,  in  primates,  117 

structure  of,  345 
Amniota,  22 
Amphibia,  23 
Amphioxus,  25 
Anal  plate,  loo 
Anamniota,  22 
Angioblast,  early  condition,  88 

of  chick,  295 

origin  of,  74 
Anlage,  definition  of,  25 
Annelids,  25 
Anura,  23 
Aorta,  chick,  277,  280 

descending,  pig,  9  mm.,  227 

dorsal,  pig,  12  mm.,  201,  202 

first  appearance,  93 

median  dorsal,  chick,  280,  287 
pig,  9  mm.,  224 
pig,  17  mm.,  234 
pig,  20  mm.,  245 

muscularis  of,  233 

origin  of  adult,  164 

pig,  24  mm.,  267 

Aortic  arches,  chick,  275,  276,  277,  280 
fourth,  pig,  9  mm.,  218 


Aortic  arches,  course  of,  194 

embryo  with  two,  136 
embryo  with  five,  138 
general  account,  164 
pig,  9  mm.,  227 
pig,  12  mm.,  189 
Appendages,  embryonic,  78 
Arachnoid,  pig,  12  mm.,  180 
pig,  17  mm.,  233 
pig,  20  mm.,  242,  254 
Arch  of  vertebra,  pig,  20  mm.,  244 
Area  opaca,  defined,  87 
of  chick,  273 
pellucida,  defined,  86 
of  chick,  272,  295 
vasculosa,  88 

circulation  of,  97 
main  vessels  of,  97 
of  chick,  297 
.     vitellina,  defined,  87 
Arteries,  allantoic,  chick,  287 

basilar,  pig,  izjnrn.,  207,  213 
carotid,  general  account,"  164 

pig,  12  mm.,  178,  180 
caudal,  247 
central,  of  retina,  263 
innominate,  168 
intersegmental,  215,  219 

pig,  6  mm.,  231 
lingual,  259 

omphalo-mesaraic,  chick,  273,  287 
pulmonary,  pig,  12  mm.,  196 

pig,  20  mm.,  247 
sulci,  242,  247 
umbilical,  pig,  17  mm  ,  239 
vertebral,  pig,  12  mm.,  191 

pig,  24  mm.,  266 
vitelline,  chick,  273,  287 

pig,  9  mm.,  221 
Ascending  trigeminal  tract,  177 
Atriozoa,  25 
Auditory  invaginations,  274,    275 

vesicle.     See  Otocyst. 
Auricle,  right,  pig,  12  mm.,  198 
Auricular  canal,  199    , 
Axis,  primitive,  65 

structural,  19 


371 


372 


INDEX. 


B. 

Bacilliform  bodies,  307 

Beale's  carmine,  369 

Biogenetic  law,  43 

Blastocyst.     See  Blastodermic  vesicles. 

Blastodermic  vesicles,  60 

fluid  contents  of,  6l 
hypothetical  development,  116 
in  alcohol,  306 
in  primates,  116 
inner  layer  of,  60 
inner  mass  of,  61 
outer  layer  of,  60 
.  rabbit,  305 

five  days,  306 
six  days,  308 
seven  days,  309 
subzonal  layer  of,  58 
trophoblast  of,  61 
Blastomeres,  isolated,  28 
Blastopore,  66 
Blind,  animals  born,  260 
Blood,  chick,  295 
origin  of,  90 
pig,  12  mm.,  181 
production  of,  in  liver,  252 
Blood-corpuscles,  93 
Blood-islands,  91 
Blood-spaces,  intervillous,  of  man,  126 

of  monkey,  122 
Blood-vessels, -definition  of,  90 
development  of,  in  chick,  91 
development  of,  in  mammals,  92 
first,  88 

growth  into  the  embryo,  92 
large  size  of,  164 
origin  of,  90 
structure  of,  chick,  295 
Body,  general  morphology,  22 
Body-cavity  (splanchnocoele),  82 
of  vertebrates,  20 
primitive,  35 

Body-stalk,  anlage  of,  117 
of  gibbon,  127 

relation  to  umbilical  cord,  109 
relations  of  allantois  to,  103 
vessels  of,  104 
Body-wall,  closure  of,  in  chick,  287 

of  vertebrates,  22 
Borax  carmine,  367 
Bern's  method  of  reconstruction,  358 
Bouchon  vaginal,  312 
Brachial  plexus,  pig,  12  mm.,  196,  199 
Brain,  chick,  in  surface  view,  273 
first  differentiation,  73 
pig,  9  mm.,  222 
pig,  12  mm.,  175 

in  sagittal  section,  205 
pig,  24  mm.,  263 

Branchial  arches.     See  Gill  arches. 
Bronchi,  pig,  17  mm.,  235 


C. 

Caduca,  118 

Canal,  auricular,  199 

hyaloid,  of  eye,  263 

of  Schlemm,  262 
Carmine,  Beale's,  369 

borax,  367 
Carnivora,  25 

Carotids,  general  account,  l6fa. 
Cartilage,  244 

origin  of,  233 

sphenoidal,  265 

Cavernous  layer  of  decidua,  328,  335 
Cavity,  hyoid,  83 

mandibular,  83 

premandibular,  83 
Cell-death,  direct,  31 

indirect,  31 
Cells,  early  condition,  chick,  293 

embryonic,  33 

germ,  defined,  26 

mass,  intermediate,  80 

removal  of,  32 

somatic,  27 

Central  artery  of  retina,  263 
Cephalochorda,  25 
Cerebellum,  pig,  12  mm.,  207 

pig,  24  mm.,  266 
Cervical  sinus,  142,  143,  144 
in  pig,  161 
pig,  9  mm.,  217 
pig,  12  mm.,  191,  208 
Chamber,  anterior,  of  eye,  261 
Cheiroptera,  25 
Chiasma,  optic,  anlage  of,  207 

pig,  24  mm.,  265 
Chick  compared  with  rabbit,  297 

histological  differentiation  of,  293 

methods  of  obtaining,  269 

methods  of  preservation,  271 

section  of,  longitudinal,  297 

serial  sections  of,  271 

with  seven  segments,  295 

with  twenty-four  segments,  272 

with  twenty-eight  segments,  sections  of,  274 

with  three  gill  clefts,  272 
Choanse,  internal,  193 
Chondrostyle,  266 
Chorda  dorsalis,  67 
Chorion,  anlage  of,  78 

chick,  282 

frondosum,  121 

human,  with  trophoblast,  342 

laeve,  121 

united  with  decidua  reflexa,  326 
united  with  decidua  vera,  327 

mesoderm  of,  333,  342 

structure  of,  331,  341 
Chorionic  villi,  general  account,  345 
Choroid  plexus  of  hind-brain,  266 
of  lateral  ventricles,  257 
pig,  24  mm.,  265 
Chromosomes,  number  of,  54 


INDEX. 


373 


Cinerea,  177 

pig,  17  mm.,  234 
Cloaca,  pig,  9  mm.,  221 

pig,  17  mm.,  239 
Ccelom,  defined,  35 

division  of,  79 
chick,  283 

extra-embryonic,  76^ 

of  head,  83 

origin  of,  75 

pericardia!,  76,  96 
chick,  297 
pig,  12  mm.,  195,  198 

pockets  of,  chick,  289 

umbilical,  pig,  9  mm.,  226 
pig,  17  mm.,  239 

ventral,  82 
Commissure,  ganglionic,  177 

posterior,  265 

superior,  265 

Compact  layer  of  decidua,  328,  335 
Connective-tissue  stain,  Mallory's,  370 
Continuity,  theory  of  germinal,  40 
Cord,  umbilical,  placental  insertion  of,  339 

structure  of,  351 
Cornea,  259 

pig,  24  mm.,  261 
Corona  radiata,  45 
Corpora  quadrigemina,  207 
Corpus  luteum,  46 

striatum,  258 
Costal  processes,  233 
Coste's  embryo,  134 
Cotyledons  of  placenta,  337 
Counterstains,  368 
Cutis,  pig,  17  mm.,  232 

pig,  20  mm.,  250 

plate,  pig,  6  mm.,  230 
Cytomorphosis,  27 


D. 

Darwin's  theory  of  pangenesis,  41 
Death  of  cells,  31 
Decidua  menstrualis,  317 
reflexa,  320,  325 
anlage  of,  118 
at  two  months,  326 
at  three  months,  326 
defined,  119 
serotina,  320,  334 
defined,  119 
subchorialis,  336 
vera,  320,  322,  327 

defined,  119 
Decidual  cells,  336 

defined, 118 
formation  of,  324 
Deck-plate,  74 
Degeneration,  31 
Development,  summary  of,  25 
types  of,  32 


Diaphragm,  anlage  of,  82 
pig,  20  mm.,  252 
pig,  24  mm.,  267 
Diencephalon,  pig,  9  mm.,  222 

pig,  12  mm.,  205 
Differentiation,  29 

in  chick  with  three  gill-clefts,  293 
types  of,  30 
Dilatations,  37 
Dipnoi,  23 
Discus  proligerus,  45 
Diverticula,  37 

Dorsal  flexure  of  embryo,  136 
furrow,  70 

zone  of  medulla,  pig,  9  mm. ,  227 
of  medulla  oblongata,  21 1 
of  medullary  canal,  74 
pig,  12  mm.,  184 
Ducts  of  Cuvier,  141 

chick,  98,  282 
pig,  12  mm.,  198 
pig,  20  mm.,  251 
urogenital,  21 
Ductus  arteriosus,  1 68 

endolymphaticus,  177 
pig,  12  mm.,  208 
venosus,  pig,  9  mm.,  221 

pig,  20  mm.,  251 
Duodenum,  pig,  24  mm.,  267 


E. 

Ectoderm,  defined,  26 

structure  of,  chick,  293 
Ectoglia,  177 

pig,  20  mm.,  242 
Eggs  of  frog,  inverted,  28 
Elasmobranchs,  23 

Embryo,  general  homologies  of  vertebrate,  77 
growth  of,  107 
half,  28 
human,  calculation  of  the  age  of,  112 

classification  of  early  stages,  113 

Nackengrube,  146,  150 

relations  to  the  uterus,  118 

umbilical  coelom,  intestines  in,  151 

twenty-eight  days,  145 

thirty-six  days,  148 

forty  days,  149 

fifty  days,  150 

sixty  days,  151 

seventy-five  days,  152 

three  months,  153 

four  months,  156 

See  also  Human  Embryo. 
measuring,  356 
orientation  for  sections,  360 
primitive  type,  77 

resemblance  to  lower  adult  forms,  43 
separation  from  yolk,  109 
serial  sections  of,  360 
shrinkage  in  paraffin,  175 


374 


INDEX. 


Embryology,  summary  of,  25 
Embryonic  cells,  33 

shield,  62 

in  section,  310,  311 
rabbit,  307,  308,  309 

type  of  development,  32 
Endothelium  of  heart,  origin,  301 

vascular,  chick,  280 
Entoderm,  defined,  26 

permanent,  67 

structure  of,  chick,  293 
Eosin,  368 

Ependymal  roof,  hind-brain,  2,07 
of  fourth  ventricle,  21 1 
Epiblast,  (foot-note)26 
Epidermis,  pig,  12  mm.,  181 

pig,  17  mm.,  232 

pig,  20  mm.,  249 
Epiglottis,  pig,  12  mm.,  207 

pig,  24  mm.,  267 
Epiphysis,  263 
Epithelium,  germinal,  39 

pericardial,  198 
Epitrichium,  pig,  12  mm.,  181 

pig,  17  mm.,  232 

Excretory  organs,  general  account,  101 
External  form  of  pig-embryos,  160,  170,  171 
Eye,  chick,  in  surface  view,  273 

pig,  20  mm.,  258 
muscles  of,  258 

pig,  24  mm  ,  section  of,  259 

rabbit  embryo  of  13  days,  259 
Eyelids,  260 

F. 

Facial  nerve,  pig,  12  mm.,  213 
Falx,  anlage  of,  190 

pig,  20 mm.,  257 
Fertilization.     See  Impregnation. 
Fin-fold,  homologue  of,  226 
Fissure,  anterior,  of  spinal  cord,  242 
Flemming's  fluid,  365 
Floor-plate,  74 

of  fourth  ventricle,  211 
Foetus  in  utero,  319,  321 
Folds,  turbinal,  255,  256 
Foramen  of  Monro,  pig,  20  mm.,  258 

of  Winslow,  205 
Fore-brain,  chick,  275,  276,  277,  280 

origin,  73 

pig,  9  mm.,  219,  222 

pig,  12  mm.,  186 

pig,  20  mm.,  256 
Fore-gut,  chick,  297,  300 
Fourth  ventricle  of  brain,  175- 

pig,  12  mm.,  211 
Fovea  cardiaca,  chick,  295 

G. 

Gall-bladder,  pig,  9  mm.,  221 
pig,  12  mm.,  204 
pig,  20  mm.,  253 


Ganglia,  cephalic,  pig,  12  mm.,  176,  2t>8 
in  frontal  section,  pig,  12  mm.,  215 
in  sagittal  section,  pig,  9  mm. ,  219 
spinal,  descent  of,  242 
pig,  17  mm.,  234 

Ganglion,  acustico-faciale,  pig,  12  mm.,  208 
acusticum,  pig,  12  mm.,  210,  214 
cells,  pig,  12  mm.,  184 
faciale,  pig,  12  mm.,  210,  214 
geniculatum,  pig,  12  mm.,  210,  213 
jugulare,  pig,  12  mm.,  210 
nodosum,  189,  193 
pig,  9  mm. ,  227 
pig,  12  mm.,  208,  210 
petrosum,  pig,  12  mm.,  208,  210 
trigeminal,  pig,  12  mm.,  213 
vestibulare,  pig,  12  mm.,  210,  213 
Ganglionic  commissure,  177 
Ganoids,  23 
Gemmules,  41  ' 

General  anatomy  of  pig  embryo,  162 
General  conceptions,  17 
Genetic  restriction,  30 
Genital  gland,  anlage  of,  204 
pig,  17  mm.,  236 
pig,  20  mm.,  251 
Germ-cells,  defined,  26 

general  account,  39,  84 
Germ-layers,  defined,  26 
general  account,  34 
specific  quality  of,  35 
Germ-plasm,  40 
Germinal  area,  97 

chick,  295 
continuity,  40 
epithelium,  defined,  39 
wall,  87 

Gibbon,  third  stage  of,  127 
Gill-arches,  chick,  in  surface  view,  273 
of  pig,  161 

pig,  9  mm.,  217,  227 
Gill-cleft,  chick,  first,  275,  276 
second,  276,  277,  280 
third,  280 

closing  plate,  275,  280 
closing  plate  of,  186,  191 
pig,  9  mm.,  219,  227 

closing  plate  of,  219 
pig,  12  mm.,  first,  184 
second,  187 
third,  190 
fourth,  193 
Glands,  definition  of,  37 

infundibular,  pig,  12  mm.,  183 
uterine,  degeneration  of,  323 
Globules,  polar,  47 
Gray  layer.      See  Cinerea. 
Growth,  law  of  unequal,  38 


H. 


Half-embryos,  28 
Hardening  methods,  363 


INDEX. 


375 


Head  bend  in  pig,  161 

process,  65 
Heart,  chick,  292 

anlage  of,  301 
aortic  end,  278 
endothelial,  278,  280,  282 
in  surface  view,  273 
muscular,  278,  280,  282 
venous  end,  280 
ventricle,  282 
early  development,  301 
endothelial,  early  human,  136,  138 
origin  of,  96 

Heidenhain's  iron  hematoxylin,  368 
Hematoxylin,  iron,  368 

Weigert's,  369 
Hemisp"heres,  pig,  12  mm.,  anlages  of,  186 

pig,  20  mm.,  254,  257 
Hensen's  knot.     See  Primitive  knot. 
Hepatic  cylinders,  loo 
Heredity,  40 
Hermann's  fluid,  365 
Hind-brain,  chick,  276,  277,  278,  280 

origin,  73 
pig,  9  mm.,  227 

Histological  differentiation  of  chick,  293 
Holoblastic  ovum,  26 
Human  embryo,  second  stage,  123 
fourth  stage,   129 
fifth  stage,  131 
sixth  stage,  132 
eighth  stage,  135 
ninth  stage,  138 
tenth  stage,  142 
eleventh  stage,  142 
twenty-six  days,  143 
twenty-seven  days,  144 
See  also  Embryo,  human. 
Humor,  vitreous,  pig,  24mm.,  263 
Hyaloid  canal  of  eye,  263 
Hyoid  branchial  arch,  161 

cavity,  83 

Hypoblast,  (foot-note)  26 
Hypophysis,  development,  207 
of  pig,  24  mm.,  265 
of  vertebrates,  21 

I. 

Ichthyopsida,  23 
Ileum,  pig,  9  mm.,  226 
Imbedding,  365 

Implantation,  defined,  106,  118 
Impregnation  in  Ascaris,  54 

in  mouse,  53,  312 

in  rabbit,  52 

site  of,  50 
Incubator,  269 
Infundibular  gland,  pig,  12  mm.,  183,  207 

pig,  24  mm.,  265 
Inner  layer  of  blastodermic  vesicle,  60 

mass  of  blastodermic  vesicle,  58,  61 
Insectivora,  25 
Intermediate  cell-mass,  80 


Intestine,  chick,  caudal,  289 

open,  285,  286,  287,  288 
pig,  9  mm.,  221 
large,  226 
structure,  226 
pig,  17  mm.,  237 
pig,  20  mm. ,  253 

large,  247 
vertebrate,  21 
Invaginations,  37 
Iris,  262 

Isthmus  of  brain,  pig,  12  mm.,  205 
pig,  24  mm.,  265 

J- 

Jakobson's  organ,  pig,  20  mm.,  255 


K. 

Kidney,  fcetal,  defined,  102 
origin  of,  237 
pig,  17  mm.,  237 
pig,  20 mm.,  251 
true,  103 

Kollmann's  embryo,  132 


L. 

Labyrinth  of  ear.     See  Otocyst. 
Lachrymal  groove,  161 

pig,  12  mm.,  190 
Lamina  postoptica,  207 

terminalis,  207 

pig,  24  mm.,  265 
Larval  type  of  development,  32 
Larynx,  pig,  12  mm.,  191,  208 
Lateral  recess  of  hind-brain,  208 

roots  of  cephalic  nerves,  212 
Layership,  defined,  36 
Lens,  anlage  of,  chick,  276 

pig,  12  mm.,  187 

pig,  24  mm.,  262 
Leucocytes,  94 
Limb-buds,  142,  162,  171 

pig,  9  mm.,  226 

pig,  12  mm.,  195 
Limbs,  anlage  of  muscles  of,  195 

of  vertebrates,  20 

posterior,  pig,  20  mm. ,  247 
Liver,  blood  production  in,  252 

chick,  283 

early  human,  140 

general  account,  IOO 

of  vertebrates,  21 

pig,  12  mm.,  204 

pig,  20  mm.,  251 

pig,  24  mm.,  267 

position  of,  21 
Lizards,  23 
Lumbar  plexus,  247 


376 


INDEX. 


Lungs,  anlages  of,  201 

early  human,  144 

pig,  9mm.,  221 

pig,  lymm.,  234 

pig,  20  mm.,  244,  246,  250 

pig,  24  mm.,  267 
Lutein,  47 
Lyons  blue,  368 

M. 

Malleus,  origin  of,  259 
Mallory's  stain,  370 
Mammae,  anlage  of,  249 
Mammals,  23 

early  development,  44 
Mammary  anlage,  171,  249 

bodies,  207 
Mandible,  pig,  9  mm.,  219 

pig,  20  mm.,  254,  255,  256 
Mandibular  cavity,  83 

process,  161 

chick,  292 
Marsipobranchs,  23 
Marsupials,  24 
Mass  and  surface,  37 
Maturation  of  the  ovum,  47 
mouse,  312 
time  of,  49 
Maxillary  process  in  pig,  161 

pig.  2O  mm.,  254,  256 
Maxillo-turbinal  fold,  240,  255 
Measuring  embryos,  356 
Meatus,  external  auditory,  anlage  of,  186 
Meckel's  cartilage,  259 
Mediastinum,  246 

Medulla  oblongata,  pig,  12  mm.,  207,  212 
Medullary  canal,  71 

structure  of,  71 

groove,  70 

chick,  274,  303 
closure  of,  chick,  296 
human,  130 

plate,  69 

human,  129 

wall,  layers  of,  227,  242,  244 

three  layers  of,  pig,  12  mm.,  183 
Membrana  decidua,  118 

serosa,  86 

Membrane,  vitelline,  defined,  40 
Menstruation,  316 
Meroblastic  ovum,  26 
Mesenchyma,  chick,  277 

defined,  34 

differentiation  of,  pig,  17  mm.,  233 

general  account,  83 

pig,  12  mm.,  181 

stages  of,  83 
Mesentery,  pig,  17  mm.,  237 

vertebrate,  21 
Mesoblast,  (foot-note)  26 
Mesoderm,  chick,  286,  288,  290 

defined,  26 

delamination  of,  64,  74 


Mesoderm,  early  history,  74 

expansion  of,  65,  74,  75 

origin  of,  63 
rabbit,  309 

somatic,  76 

splanchnic,  76 

structure  of,  chick,  294 
Mesogastrium,  204 
Mesonephros.     See  Wolffian  bodies. 
Mesothelium,  defined,  35 

of  heart,  chick,  301 

origin  of,  76,  82 

pig,  9  mm.,  224,  225 
Metanephros,  defined,  103 
Methods,  general,  356 
Microtomes,  360 
Mid-brain,  origin,  73 

pig,  9  mm.,  222 

pig,  24  mm.,  266 
Milk-gland.     See  Mamma. 
Milk-line,  162 

Monkey,  second  stage  of,  121 
Monotremes,  23 
Mounting  paraffin  sections,  365 
Mouth,  pig,  12  mm.,  207 

vertebrate,  20 

Mouth-cavity,  pig,  20  mm.,,  240 
Miiller's  fluid,  364 
Muscle-plate,  pig,  6  mm.,  231 

pig,  9  mm.,  223 
Muscles,  dorsal,  pig,  12  mm.,  199 

hyoglossal,  259 
Myotome,  origin  of,  82 

pig,  9  mm.,  217 

pig,  12  mm.,  187 


N. 

Nasal  pits,  pig,  12  mm.,  187,  193 
septum,  pig,  20  mm.,  241,  255 

Naso-turbinal  fold,  255 

Neck  bend  in  pig,  160 

Necrobiosis,  31 

Nephrotome,  chick,  287 
origin  of,  80 
separation  of,  81 

Nerve,  facial,  roots  of,  215 
fourth,  origin,  205 
maxillary,  pig,  20  mm.,  255 
oculomotor,  pig,  12  mm.,  180 
olfactory,  pig,  20  mm.,  255 
optic,  pig,  12  mm.,  210 
roots,  lateral,  184 

pig,  6  mm.,  230 
structure,  pig,  12  mm.,  215 
trochlear,  pig,  12  mm.,  180 

Nerves,  cephalic,  pig,  12  mm.,  186,  210 
cervical,  first,  187 

second,  191 
post-trematic,  190 
spinal,  morphology  of,  196 
pig,  17  mm.,  234 


INDEX. 


377 


Nervous  system,  hollow,  20 

origin  of,  69 
Neural  crest,  chick,  299 
Neuraxons,  pig,  12  mm.,  184 
Neurenteric  canal,  70 
of  gibbon,  128 
of  man,  130 
Neuromeres,  chick,  292 

pig,  6  mm.,  229 

pig,  12  mm.,  178 
Neurone  layer.     See  Cinerea. 
Neuropore,  anterior,  71.  29^ 
Notochord,  67 

anlage  of,  in  gibbon,  128 

chick,  278,  288,  289,  290 

pig,  12  mm.,  187 

pig,  17  mm.,  233 

pig,  20  mm.,  246 

pig,  24  mm.,  266 

ultimate  fate  of,  68 

vertebrate,  19 

young  chick,  299,  303 
Notochordal  canal,  66 
Nussbaum's  theory,  40 


O. 

(Esophagus,  pig,  12  mm.,  196,  201 

pig,  17  mm.,  233,  234 

pig,  20  mm.,  242,  246 

pig,  24  mm.,  267 
Olfactory  nerve,  development,  255 

pits.     See  Nasal  pits. 

plate,  pig,  9  mm.,  219 
pig,  12  mm.,  193 
Omentum,  great,  204 

lesser,  204 
Operculum,  227 
Optic  chiasma,  anlage  of,  207 
pig,  24  mm.,  265 

nerve,  chick,  292 

stalks,  pig,  12  mm.,  186 

vesicles,  chick,  297 

primary,  73 
Oral  cavity,  pig,  20  mm.,  240 

fissure,  pig,  12  mm.,  187 

plate,  100 
Orange  G,  368 
Organ  of  Jakobson,  255 
Organs,  constitution  of,  36 
Otocyst,  chick,  in  section,  274 
in  surface  view,  273 

pig,  6  mm. ,  229 

pig,  12  mm.,  176,  177,  208,  215 

in  frontal  section,  214 
Ovulation,  46 
Ovum,  arrival  in  uterus,  59 

before  maturation,  45 

entrance  of  spermatozoon  into,  50 

holoblastic,  26 

human,  45 

impregnation  of,  49 


Ovum,  maturation  of,  47 
meroblastic,  26 
of  mouse,  312 

method  of  obtaining,  312 
segmentation  of  mammalian,  54 


P. 

Palate,  cleft,  241 

shelf,  pig,  20  mm.,  240 
Panchoroid,  pig,  12  mm.,  181 

pig,  20  mm.,  250 
Pancreas,  pig,  24  mm.,  267 
Pangenesis,  41 
Parathyroid  glands,  244 
Parietal  zone,  79 
Parker's  fluid,  364 
Peduncles  of  brain,  207 
Penis,  247 

Pericardial  membrane,  198 
Perichondrium,  origin  of,  233 
Peritoneum,  202 

pig,  20  mm.,  250 
Peters's  ovum,  123 
Pharynx,  chick,  276,  277,  278,  281 

early  human,  144 

pig,  12  mm.,  184 

vertebrate,  19 

young  chick,  299 
Pia  mater,  pig,  9  mm.,  222 
pig,  12  mm.,  180 
pig,  1 7  mm.,  233 
pig,  20  mm..  253 
Picro-sulphuric  acid,  364 
Pig,  6  mm.,  sections  of,  228 

9  mm.,  sections  of,  217 

10  mm.,  external  form,  160 
12  mm.,  brain  of,  175,  205 

diagram  of  planes  of  sections,  174 
frontal  sections  of,  211 
general  anatomy  of,  162 
sagittal  sections  of,  205 
studied  in  sections,  173 
transverse  sections  of,  175 
15  mm.,  external  form,  170 
17  mm.,  sections  of,  231 
20  mm.,  sections  of,  240 
24  mm.,  median  sagittal  section  of,  263 

sections  of,  259 
embryos,  157 

external  form,  159 
methods  of  obtaining,  157 
planes  of  sections  of,  158 
reconstructions  of,  162 
selection  of  stages,  159 
serial  sections  of,  158 
Pigment  layer  of  eye,  262 
Pineal  organ,  263 
Pituitary  body,  183 

pig,  24  mm.,  265 
Placenta  after  delivery,  336 
at  seven  months,  321 
cotyledons  of,  337 


378 


INDEX. 


Placenta,  foetal  blood  supply  of,  339 
in  situ,  329 

maternal  blood  supply  of,  341 
PJacentalia,  24 
Plakodes,  chick,  293 

olfactory,  pig,  9  mm.,  219 
Plate,  anal  and  oral,  100 
Pleural  cavity,  pig,  17  mm.,  234 

pig,  20  mm.,  251 
Pleuro-peritoneal  space,  82 
Plexus,  brachial,  pig,  12  mm.,  196,  199 
choroid  of  hind-brain,  266 
of  lateral  ventricle,  257 
pig,  24  mm.,  265 
lumbar,  pig,  20  mm.,  247 
Polar  globules,  47 

mouse,  312 

formation  of  first,  313 
formation  of  second,  313 
formation  of  single1,  314 
Post-optic  lamina,  207 
Premandibular  cavity,  83 
Preservation  in  alcohol,  365 
Preserving  methods,  363 
Primates,  25 
Primitive  axis,  65 
body-cavity,  20 
groove,  chick,  274,  296 
first  appearance,  62 
knot,  62 

rabbit,  309,  310 
streak,  chick,  297,  303 
first  appearance,  62 
obliteration  of,  65 
rabbit,  310 
structural  axis,  19 
Pro-amnion,  chick,  279,  295 
origin  of,  75 
young  chick,  300 
Processes,  costal,  233 
Pro-chorion,  59 
Projections,  morphological,  37 
Pronephros,  morphology,  101 
Pro-nuclei,  fusion  of,  52 
Pro-nucleus,  female,  49 

mouse,  315 
male,  51 
Prosencephalon,  pig,  9  mm.,  222 

pig,  12  mm.,  205 
Proto-vertebrse,  80 
Pupil  of  eye,  262 

R. 

Raphe  of  amnion,  chick,  278,  282,  284 

of  medulla,  180 

of  medulla  oblongata,  211 
Rauber's  layer,  307,  310 
Recapitulation,  law  of,  41 
Recessus  lateralis  of  hind-brain,  208 
Reconstruction,  methods  of,  356 

by  drawings,  356 

by  wax  plates,  358 


Reduction  division,  47 

Regression  of  cells,  31 

Renal  anlage,  238 

Reptiles,  23 

Restriction,  genetic,  30 

Retina,  anlage  of,  chick,  276,  277 

pig,  12  mm.,  186 

pig,  24  mm.,  262 
Rhomboidal  sinus,  296 
Ribs,  anlages  of,  233 

pig,  20  mm.,  250 
Ridge,  urogenital,  21 
Rodents,  25 
Roots,  lateral,  of  cephalic  nerves,  211 

of  nerves,  ventral,  196 
dorsal,   196 

S. 

Sauropsida,  23 
Scapula,  244 
Schlemm,  canal  of,  262 
Sclerotome,  pig,  6  mm.,  230 
Segmental  vesicle,  81 
zone,  79 

chick,  288,  292 

Segmentation,  two-cell  stage,  57 
four-cell  stage,  58 
of  Limax  ovum, "55 
of  mammalian  ovum,  54 
of  the  ovum,  defined,  25 

mouse,  312 
Segments,  chick,  287,  292 

in  surface  view,  274 
secondary,  283 
external,  of  pig,  162 
pig,  6  mm.,  230 
primitive  79 

chick,  296,  304 
secondary,  8 1 

pig,  6  mm.,  231 
pig,  9  mm.,  217 
Sella  turcica,  265 
Sense  organs  of  vertebrates,  21 
Septum,  nasal.     See  Nasal  septum. 
transversum,  chick,  283 
origin  of,  82 
pig,  20  mm.,  252 
Sexual  cords,  251 
Shield,  embryonic,  62 

rabbit,  307,  308,  309 

in  section,  310,  311 
Shrinking  of  embryo  in  paraffin,  175 
Sinus,  cervical,  pig,  12  mm.,  191 
•     rhomboidalis,  296 

superior  longitudinal,  180,  190 
terminalis  of  chick,  91,  98,  273 

of  rabbit,  loo 
venosus,  chick,  98,  283 
Sinusoid  of  heart,  199 

pig,  9  mm.,  221 
of  liver,  204 

chick,  283 
of  Wolffian  body,  204,  221,  225,  235 


INDEX. 


379 


Skin,  outer,  pig,  1 2  mm.,  181 
Skull,  anlage  of  membranous,  254 
Snout,  pig,  20  mm.,  254,  255,  256 

section  of,  240 
Somatic  cavity,  82 

cells,  27 
Somatopleure,  76 

extra-embryonic,  chick,  286 

pig,  12  mm.,  202 

vertebrate,  22 
Special  sense  organs,  21 
Spec's  embryo,  129 
Spermatozoon,  44 

entrance  of,  into  ovum,  50 
Sphenoid  cartilage,  265 
Spinal  cord,  anterior  fissure,  242 
chick,  280,  288 
differentiation  from  brain,  73 
pig,  6  mm.,  230 
pig,  17  mm.,  233 
pig,  2O  mm.,  241 
pig,  24  mm. ,  266 

ganglia,  descent  of,  242 

nerves.     See  Nerves,  spinal. 
Splanchnocoele,  defined,  79 

origin  of,  82 
Splanchnopleure,  76 

of  vertebrates,  22 
Staining,  counterstaining,  368 

methods  of,  367 
Stomach,  204 

pig,  24  mm.,  267 

vertebrate,  21 

Streak,  primitive,  chick,  297 
Striae  acusticse,  pig,  12  mm.,  183 
Structure,  vertebrate  type  of,  18 
Subzonal  layer,  58,  307,  308 
Suprarenal,  development,  267 

pig,  24  mm.,  267 
Surface  and  mass,  37 
Sympathetic,  cervical,  pig,  12  mm.,  194 

pig,  17  mm.,  234 

pig,  2O  mm.,  242 

pig,  24  mm.,  267 

thoracic,  pig,  12  mm.,  201 


T. 

Teleosts,  23 

Tellyesnicky's  fluid,  364 
Terminal  sinus  of  chick,  273 
Testis,  pig,  24  mm.,  267 
Thymus,  pig,  12  mm.,  191,  209 
Thyroid,  pig,  12  mm.,  208 
median,  191 

pig,  20  mm.,  244 
Tissues,  classification  of,  35 
Tongue,  anlage  of,  140,  207 

pig,  20  mm.,  240,  255,  259 

pig,  24  mm.,  267 
Trachea,  pig,  12  mm.,  191,  195,  201 

pig,  20  mm.,  242,  246 
Tractus  solitarius,  213 


Trigeminal  nerve,  pig,  12  mm.,  212 

tracts,  177 
Trophoblast,  general  account,  106 

human,  342 

in  primates,  Il6 

of  human  embryo  of  the  second  stage,  125 

of  monkey,  122 
Tuber  cinereum,  207 
Tuberculum  impar,  140 
Tunica  albuginea,  251 

vasculosa  of  eye,  259 
Tunicata,  25 
Turbinal  folds,  255,  256 


U. 

Umbilical  coelom,  pig,  9  mm.,  226 

pig,  17  mm.,  239 
cord,  320,  322 

connection  with  chorion,  121 
general  account,  109 
pig,  section  of,  239 
placental  insertion  of,  339 
structure  of,  351 

opening,  pig,  9  mm.,  221,  223,  224 
Undifferentiated  stage,  27 
Unguiculates,  24 
Ungulates,  24 
Urethra,  247 
Urodela,  23 
Urogenital  ducts,  21 

ridge,  21 

Uterine  glands,  degeneration  of,  323 
Uterus,  histology  of,  316 

pregnant,  general  account,  119 

two  stages  of,  319 
three  months  pregnant,  319 
seven  months  pregnant,  321 
Uvea,  pig,  24  mm.,  262 


V. 

Vagus  nerve,  pig,  17  mm.,  234 
pig,  20  mm.,  244,  246 
Valves,  atrio- ventricular,  pig,  12  mm.,  199 
Eustachian,  198 
Thebesian,  198 
venous  of  right  auricle,  198 
Vasoformative  cells,  93 
Veins,  allantoic.  141 

anterior  cardinal,  pig,  183 
pig,  12  mm.,  213 
axillary,  pig,  12  mm.,  196 
cardinals.  141 

chick,  98,  280,  282,  284 
pig,  9  mm. ,  225 
pig,  12  mm.,  201 
pig,  17  mm.,  234 
cava  inferior,  origin  of,  226 
pig,  12  mm.,  204 
early  disposition  of,  168 
first  embryonic,  14! 


380 


INDEX. 


Veins,  iliac,  pig,  20  mm.,  247 
inferior  maxillary,  208 
intersegmental,  215 
jugular,  of  chick,  98 

pig,  12  mm.,  183,  210,  214 
lateral  jugular,  1 80 
mesonephric,  221 

pig,  17  mm.,  235 

pig,  20  mm.,  251 
omphalo-mesaraic,  93,  141 

chick,  98,  285 
peripheral,  of  limbs,  247 
portal,  pig,  9  mm.,  221 
posterior  cardinal,  2OI 

pig,  20  mm.,  245 
pulmonary,  20 1 
subcardinal,  pig,  9  mm.,  226 
subclavian,  pig,  12  mm.,  196 

pig,  17  mm.,  234 
umbilical,  early  human,  136 

pig,  9  mm.,  226 

pig,  12  mm.,  204 

pig,  17  mm.,  239 
vitelline,  pig,  17  mm.,  239 
Ventral  roots,  186 

zone,  of  medulla,  pig,  9  mm.,  227 

of  medulla  oblongata,  211 

of  medullary  canal,  74 

pig,  12  mm.,  184 
Ventricle,  fourth,  of  brain,  175 
chick,  290 
pig,  24  mm.,  266 
lateral,  pig,  12  mm.,  186 

pig,  20  mm.,  256 
of  heart,  pig,  12  mm.,  199 
Vertebrae,  pig,  12  mm.,  202,  215 

pig,  I?  mm-,  233 
pig,  20  mm.,  242 
pig,  24  mm.,  266 
Vertebral  arch,  pig,  20  mm.,  244 
Vertebrate  type,  18 

modifications  of,  22 

Vesicles,  amnio-cardiac,  chick,  295,  300 
blastodermic,  60 
in  alcohol,  306 
rabbit,  305 

five  days,  306 
six  days,  308 
seven  days,  309 
definition  of,  37 
optic,  chick,  276,  277,  297 
primary,  73 


Vesicles,  primary  cerebral,  73 

chick,  273 
segmental,  8l 

Villi,  chorionic,  320,  327,  333 
early  human,  135 
general  account,  345 
of  gibbon,  127 
origin  of,  117,  124 
mesothelial  of  allantois,  222,  253 
Vitelline  membrane,  defined,  40 
Vitreous  humor,  260,  263 
pig,  24mm.,  263 


W. 

Weigert's  hematoxylin,  369 
Weismann's  hypotheses,  41 
Winslow,  foramen  of,  205 
Wolffian  bodies,  202 

division  of,  into  two  parts,  25 1 

general  account,  102 

morphology,  102 

pig,  9  mm.,  219,  225 

pig,  17  mm.,  235 

pig,  20  mm.,  251 
duct,  chick,  287 

pig,  9  mm.,  225 

pig,  17  mm.,  238 
tubules,  pig,  9  mm  ,  225 


Y. 

Yolk- cavity,  66 
Yolk-sac,  85 

anlage  of,  78 

human,  89,  320 

structure  of,  354 

of  gibbon,  127,  128 
Yolk-stalk  in  umbilical  cord,  no,  352 


Z. 

Zenker's  fluid,  363 

Zona  pellucida,  defined,  40 

rabbit,  309 
Zones  of  His,  74 

pig,  9  mm. ,  217 
pig,  12  mm.,  185,  1 86 
parietal,  79 
segmental,  79 


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